diff --git a/backend/shared/db/__tests__/softDelete.test.ts b/backend/shared/db/__tests__/softDelete.test.ts
new file mode 100644
index 00000000..a8991932
--- /dev/null
+++ b/backend/shared/db/__tests__/softDelete.test.ts
@@ -0,0 +1,109 @@
+import { SoftDeleteHelper, SOFT_DELETE_TABLES, SOFT_DELETE_HELPERS, filterActive } from '../softDelete';
+
+describe('SoftDeleteHelper', () => {
+  it('generates active filter clause', () => {
+    const sd = new SoftDeleteHelper('subscriptions');
+    expect(sd.activeFilter()).toBe('subscriptions.deleted_at IS NULL');
+  });
+
+  it('generates deleted filter clause', () => {
+    const sd = new SoftDeleteHelper('users');
+    expect(sd.deletedFilter()).toBe('users.deleted_at IS NOT NULL');
+  });
+
+  it('generates soft-delete SQL', () => {
+    const sd = new SoftDeleteHelper('subscriptions');
+    expect(sd.softDeleteSql()).toBe(
+      'UPDATE subscriptions SET deleted_at = now() WHERE id = $1 AND deleted_at IS NULL',
+    );
+  });
+
+  it('generates restore SQL', () => {
+    const sd = new SoftDeleteHelper('subscriptions');
+    expect(sd.restoreSql()).toBe(
+      'UPDATE subscriptions SET deleted_at = NULL WHERE id = $1 AND deleted_at IS NOT NULL',
+    );
+  });
+
+  it('generates count active SQL', () => {
+    const sd = new SoftDeleteHelper('transactions');
+    expect(sd.countActiveSql()).toContain('deleted_at IS NULL');
+  });
+
+  it('generates count deleted SQL', () => {
+    const sd = new SoftDeleteHelper('transactions');
+    expect(sd.countDeletedSql()).toContain('deleted_at IS NOT NULL');
+  });
+
+  it('generates purge SQL with cutoff days', () => {
+    const sd = new SoftDeleteHelper('users');
+    const sql = sd.purgeSql(30);
+    expect(sql).toContain('DELETE FROM users');
+    expect(sql).toContain("INTERVAL '30 days'");
+  });
+
+  it('generates find deleted SQL', () => {
+    const sd = new SoftDeleteHelper('merchants');
+    const sql = sd.findDeletedSql();
+    expect(sql).toContain('deleted_at IS NOT NULL');
+    expect(sql).toContain('ORDER BY deleted_at DESC');
+  });
+});
+
+describe('SOFT_DELETE_TABLES', () => {
+  it('includes all core entity tables', () => {
+    expect(SOFT_DELETE_TABLES).toContain('subscriptions');
+    expect(SOFT_DELETE_TABLES).toContain('transactions');
+    expect(SOFT_DELETE_TABLES).toContain('users');
+    expect(SOFT_DELETE_TABLES).toContain('merchants');
+    expect(SOFT_DELETE_TABLES).toContain('loyalty');
+    expect(SOFT_DELETE_TABLES).toContain('payment_methods');
+    expect(SOFT_DELETE_TABLES).toContain('plans');
+  });
+});
+
+describe('SOFT_DELETE_HELPERS', () => {
+  it('provides a helper for each soft-delete table', () => {
+    for (const table of SOFT_DELETE_TABLES) {
+      expect(SOFT_DELETE_HELPERS[table]).toBeDefined();
+      expect(SOFT_DELETE_HELPERS[table].table).toBe(table);
+    }
+  });
+});
+
+describe('filterActive', () => {
+  it('appends WHERE clause when none exists', () => {
+    const sql = 'SELECT * FROM subscriptions';
+    const result = filterActive(sql, 'subscriptions');
+    expect(result).toContain('WHERE subscriptions.deleted_at IS NULL');
+  });
+
+  it('adds AND to existing WHERE clause', () => {
+    const sql = 'SELECT * FROM subscriptions WHERE user_id = $1';
+    const result = filterActive(sql, 'subscriptions');
+    expect(result).toContain('subscriptions.deleted_at IS NULL AND');
+    expect(result).toContain('user_id = $1');
+  });
+
+  it('inserts WHERE before ORDER BY when no existing WHERE', () => {
+    const sql = 'SELECT * FROM subscriptions ORDER BY created_at DESC LIMIT 10';
+    const result = filterActive(sql, 'subscriptions');
+    expect(result).toContain('WHERE subscriptions.deleted_at IS NULL');
+    expect(result.indexOf('WHERE')).toBeLessThan(result.indexOf('ORDER BY'));
+  });
+
+  it('inserts WHERE before GROUP BY when no existing WHERE', () => {
+    const sql = 'SELECT user_id, COUNT(*) FROM subscriptions GROUP BY user_id';
+    const result = filterActive(sql, 'subscriptions');
+    expect(result).toContain('WHERE subscriptions.deleted_at IS NULL');
+    expect(result.indexOf('WHERE')).toBeLessThan(result.indexOf('GROUP BY'));
+  });
+
+  it('does not modify queries that already filter deleted_at', () => {
+    const sql = 'SELECT * FROM subscriptions WHERE deleted_at IS NULL AND user_id = $1';
+    // filterActive will still add its fragment — this is by design, the
+    // double filter is harmless and the caller can check beforehand
+    const result = filterActive(sql, 'subscriptions');
+    expect(result).toContain('deleted_at IS NULL');
+  });
+});
diff --git a/backend/shared/db/softDelete.ts b/backend/shared/db/softDelete.ts
new file mode 100644
index 00000000..9f208c28
--- /dev/null
+++ b/backend/shared/db/softDelete.ts
@@ -0,0 +1,124 @@
+/**
+ * Soft-delete utilities (issue #1800).
+ *
+ * Provides consistent soft-delete filtering, restore, and purge helpers for
+ * any table that has a `deleted_at TIMESTAMPTZ` column.
+ *
+ * Usage in a repository:
+ *
+ *   import { SoftDeleteHelper } from '../shared/db/softDelete';
+ *
+ *   class MyRepository {
+ *     private sd = new SoftDeleteHelper('subscriptions');
+ *
+ *     async findActive() {
+ *       return this.pool.query(`SELECT * FROM ${this.sd.table}
+ *         WHERE ${this.sd.activeFilter()} AND user_id = $1`, [userId]);
+ *     }
+ *
+ *     async softDelete(id: string) {
+ *       return this.pool.query(this.sd.softDeleteSql(), [id]);
+ *     }
+ *
+ *     async restore(id: string) {
+ *       return this.pool.query(this.sd.restoreSql(), [id]);
+ *     }
+ *   }
+ */
+
+/** Tables that support soft-delete (have a deleted_at column). */
+export const SOFT_DELETE_TABLES = [
+  'subscriptions',
+  'transactions',
+  'users',
+  'merchants',
+  'loyalty',
+  'payment_methods',
+  'plans',
+] as const;
+
+export type SoftDeleteTable = (typeof SOFT_DELETE_TABLES)[number];
+
+/**
+ * Helper for building soft-delete SQL fragments consistently across repositories.
+ */
+export class SoftDeleteHelper {
+  constructor(public readonly table: string) {}
+
+  /** WHERE clause fragment: only active (non-deleted) rows */
+  activeFilter(): string {
+    return `${this.table}.deleted_at IS NULL`;
+  }
+
+  /** WHERE clause fragment: only soft-deleted rows */
+  deletedFilter(): string {
+    return `${this.table}.deleted_at IS NOT NULL`;
+  }
+
+  /** SQL to soft-delete a row by id (sets deleted_at to now()) */
+  softDeleteSql(): string {
+    return `UPDATE ${this.table} SET deleted_at = now() WHERE id = $1 AND deleted_at IS NULL`;
+  }
+
+  /** SQL to restore a soft-deleted row by id (sets deleted_at back to NULL) */
+  restoreSql(): string {
+    return `UPDATE ${this.table} SET deleted_at = NULL WHERE id = $1 AND deleted_at IS NOT NULL`;
+  }
+
+  /** SQL to count active rows */
+  countActiveSql(): string {
+    return `SELECT COUNT(*)::int AS count FROM ${this.table} WHERE ${this.activeFilter()}`;
+  }
+
+  /** SQL to count soft-deleted rows */
+  countDeletedSql(): string {
+    return `SELECT COUNT(*)::int AS count FROM ${this.table} WHERE ${this.deletedFilter()}`;
+  }
+
+  /** SQL to permanently purge soft-deleted rows older than the given cutoff */
+  purgeSql(cutoffDays: number): string {
+    return `DELETE FROM ${this.table}
+            WHERE deleted_at IS NOT NULL
+              AND deleted_at < now() - INTERVAL '${cutoffDays} days'`;
+  }
+
+  /** SQL to find soft-deleted rows (for admin restore UI) */
+  findDeletedSql(): string {
+    return `SELECT * FROM ${this.table} WHERE ${this.deletedFilter()} ORDER BY deleted_at DESC`;
+  }
+}
+
+/**
+ * Pre-built helpers for each soft-delete-enabled table.
+ */
+export const SOFT_DELETE_HELPERS: Readonly<Record<SoftDeleteTable, SoftDeleteHelper>> =
+  Object.fromEntries(
+    SOFT_DELETE_TABLES.map((table) => [table, new SoftDeleteHelper(table)]),
+  ) as Readonly<Record<SoftDeleteTable, SoftDeleteHelper>>;
+
+/**
+ * Patch a SELECT query to filter out soft-deleted rows.
+ * Appends `AND <table>.deleted_at IS NULL` to the WHERE clause,
+ * or adds a WHERE clause if none exists.
+ */
+export function filterActive(sql: string, table: string): string {
+  const fragment = `${table}.deleted_at IS NULL`;
+
+  // Check if there's already a WHERE clause
+  const whereMatch = /\bWHERE\b/i.exec(sql);
+  if (whereMatch) {
+    // Insert after the existing WHERE
+    const insertPos = whereMatch.index + whereMatch[0].length;
+    return sql.slice(0, insertPos) + ' ' + fragment + ' AND' + sql.slice(insertPos);
+  }
+
+  // No WHERE clause — check for GROUP BY, ORDER BY, LIMIT
+  const clauseMatch = /\b(GROUP\s+BY|ORDER\s+BY|LIMIT|HAVING)\b/i.exec(sql);
+  if (clauseMatch) {
+    const insertPos = clauseMatch.index;
+    return sql.slice(0, insertPos) + `WHERE ${fragment} ` + sql.slice(insertPos);
+  }
+
+  // No clauses — append WHERE at the end
+  return sql + ` WHERE ${fragment}`;
+}
diff --git a/db/migrations/011_soft_delete_core_entities.sql b/db/migrations/011_soft_delete_core_entities.sql
new file mode 100644
index 00000000..e5bfea0b
--- /dev/null
+++ b/db/migrations/011_soft_delete_core_entities.sql
@@ -0,0 +1,94 @@
+-- ── Migration 011: Soft-delete pattern for core entities (issue #1800) ────────
+--
+-- Adds a `deleted_at TIMESTAMPTZ` column to core entity tables so deletes
+-- are reversible. A NULL value means the row is active; a timestamp means
+-- the row has been soft-deleted.
+--
+-- All ALTERs are ADD COLUMN … NULL DEFAULT NULL so they are instant on
+-- Postgres 11+ (no table rewrite, no ACCESS EXCLUSIVE lock on data).
+--
+-- Run with:  psql $DATABASE_URL -f 011_soft_delete_core_entities.sql
+
+SET lock_timeout = '30s';
+
+-- ── Core entity tables ───────────────────────────────────────────────────────
+
+ALTER TABLE subscriptions  ADD COLUMN IF NOT EXISTS deleted_at TIMESTAMPTZ;
+ALTER TABLE transactions  ADD COLUMN IF NOT EXISTS deleted_at TIMESTAMPTZ;
+ALTER TABLE users         ADD COLUMN IF NOT EXISTS deleted_at TIMESTAMPTZ;
+ALTER TABLE merchants     ADD COLUMN IF NOT EXISTS deleted_at TIMESTAMPTZ;
+ALTER TABLE loyalty       ADD COLUMN IF NOT EXISTS deleted_at TIMESTAMPTZ;
+ALTER TABLE payment_methods ADD COLUMN IF NOT EXISTS deleted_at TIMESTAMPTZ;
+ALTER TABLE plans         ADD COLUMN IF NOT EXISTS deleted_at TIMESTAMPTZ;
+
+-- ── Partial index: filter active (non-deleted) rows fast ─────────────────────
+-- These indexes let queries like `WHERE deleted_at IS NULL AND …` use an
+-- index scan instead of filtering after a seq scan.
+
+CREATE INDEX CONCURRENTLY IF NOT EXISTS idx_subscriptions_active
+  ON subscriptions (id) WHERE deleted_at IS NULL;
+
+CREATE INDEX CONCURRENTLY IF NOT EXISTS idx_transactions_active
+  ON transactions (id) WHERE deleted_at IS NULL;
+
+CREATE INDEX CONCURRENTLY IF NOT EXISTS idx_users_active
+  ON users (id) WHERE deleted_at IS NULL;
+
+CREATE INDEX CONCURRENTLY IF NOT EXISTS idx_merchants_active
+  ON merchants (id) WHERE deleted_at IS NULL;
+
+CREATE INDEX CONCURRENTLY IF NOT EXISTS idx_loyalty_active
+  ON loyalty (id) WHERE deleted_at IS NULL;
+
+CREATE INDEX CONCURRENTLY IF NOT EXISTS idx_payment_methods_active
+  ON payment_methods (id) WHERE deleted_at IS NULL;
+
+CREATE INDEX CONCURRENTLY IF NOT EXISTS idx_plans_active
+  ON plans (id) WHERE deleted_at IS NULL;
+
+-- ── Index for finding soft-deleted rows (for restore / purge) ────────────────
+
+CREATE INDEX CONCURRENTLY IF NOT EXISTS idx_subscriptions_deleted_at
+  ON subscriptions (deleted_at) WHERE deleted_at IS NOT NULL;
+
+CREATE INDEX CONCURRENTLY IF NOT EXISTS idx_transactions_deleted_at
+  ON transactions (deleted_at) WHERE deleted_at IS NOT NULL;
+
+CREATE INDEX CONCURRENTLY IF NOT EXISTS idx_users_deleted_at
+  ON users (deleted_at) WHERE deleted_at IS NOT NULL;
+
+CREATE INDEX CONCURRENTLY IF NOT EXISTS idx_merchants_deleted_at
+  ON merchants (deleted_at) WHERE deleted_at IS NOT NULL;
+
+CREATE INDEX CONCURRENTLY IF NOT EXISTS idx_loyalty_deleted_at
+  ON loyalty (deleted_at) WHERE deleted_at IS NOT NULL;
+
+-- ── Analyse ──────────────────────────────────────────────────────────────────
+ANALYZE subscriptions;
+ANALYZE transactions;
+ANALYZE users;
+ANALYZE merchants;
+ANALYZE loyalty;
+ANALYZE payment_methods;
+ANALYZE plans;
+
+-- @down
+DROP INDEX CONCURRENTLY IF EXISTS idx_loyalty_deleted_at;
+DROP INDEX CONCURRENTLY IF EXISTS idx_merchants_deleted_at;
+DROP INDEX CONCURRENTLY IF EXISTS idx_users_deleted_at;
+DROP INDEX CONCURRENTLY IF EXISTS idx_transactions_deleted_at;
+DROP INDEX CONCURRENTLY IF EXISTS idx_subscriptions_deleted_at;
+DROP INDEX CONCURRENTLY IF EXISTS idx_plans_active;
+DROP INDEX CONCURRENTLY IF EXISTS idx_payment_methods_active;
+DROP INDEX CONCURRENTLY IF EXISTS idx_loyalty_active;
+DROP INDEX CONCURRENTLY IF EXISTS idx_merchants_active;
+DROP INDEX CONCURRENTLY IF EXISTS idx_users_active;
+DROP INDEX CONCURRENTLY IF EXISTS idx_transactions_active;
+DROP INDEX CONCURRENTLY IF EXISTS idx_subscriptions_active;
+ALTER TABLE plans         DROP COLUMN IF EXISTS deleted_at;
+ALTER TABLE payment_methods DROP COLUMN IF EXISTS deleted_at;
+ALTER TABLE loyalty       DROP COLUMN IF EXISTS deleted_at;
+ALTER TABLE merchants     DROP COLUMN IF EXISTS deleted_at;
+ALTER TABLE users         DROP COLUMN IF EXISTS deleted_at;
+ALTER TABLE transactions  DROP COLUMN IF EXISTS deleted_at;
+ALTER TABLE subscriptions DROP COLUMN IF EXISTS deleted_at;