fix(db): declare on-delete behaviour for group relations

[gtrefe]

Overview

This PR declares an explicit onDelete behaviour for every relation hanging off SavingsGroup in prisma/schema.prisma, instead of relying on Prisma's implicit Restrict default. Operational rows (memberships, payouts) are cascaded with the group; append-only audit records (PayoutOrderLog, MemberExitLog) are preserved by making their group reference nullable with SetNull. A migration applies the new foreign-key actions, and the group deletion path plus its tests are updated to match.

Related Issue

Changes

🗄️ Schema — explicit delete policies

• [MODIFY] prisma/schema.prisma
  • GroupMember.group and Payout.group → onDelete: Cascade (operational rows have no meaning without the group).
  • PayoutOrderLog.group and MemberExitLog.group → group reference made optional with onDelete: SetNull (append-only audit records survive the group they describe).
  • Each relation now carries a short comment documenting the chosen policy.

🧩 Service — deletion path

• [MODIFY] src/services/group.service.ts
  • Group deletion no longer trips a foreign-key violation for groups that have members, payouts or exit logs; the cascade/nullify behaviour is now relied on deliberately rather than accidentally.

🧪 Tests

• [MODIFY] src/__tests__/group.service.test.ts
  • Covers the intended behaviour per relation: memberships and payouts are removed with the group, while payout order logs and member exit logs remain and have their group reference nulled.

🚚 Migration

• [ADD] prisma/migrations/20260901000000_add_group_relation_on_delete/migration.sql
  • Drops and recreates the affected foreign keys with the new ON DELETE CASCADE / ON DELETE SET NULL actions, and makes the audit-log group columns nullable.

Verification Results

npm test
✅ group.service tests pass (cascade + audit-retention cases)

Acceptance Criteria	Status
Each group relation states its delete behaviour explicitly	✅ onDelete declared on GroupMember, Payout, PayoutOrderLog, MemberExitLog
Audit records survive the deletion of the group they reference	✅ PayoutOrderLog / MemberExitLog use nullable group ref + SetNull
A test covers the intended behaviour for each relation	✅ Added to src/__tests__/group.service.test.ts
npm test passes locally	✅ Passing

Closes #248

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