feat(billing): back disputes router with Postgres repository (#1243)

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Summary

Backs the billing disputes subsystem with a PostgreSQL repository (PgDisputeRepository), mounts /api/billing/disputes in src/app.ts, and ensures dispute state and audit trails survive process restarts and work across replicas.

Closes #1243

Key Changes

1. PostgreSQL Repository (src/services/disputeService.ts):
   • Implemented PgDisputeRepository against migrations/0019_disputes.sql (disputes and dispute_events tables), using transactional BEGIN / COMMIT / ROLLBACK writes when available.
   • Updated DisputeRepository interface and DisputeService with MaybePromise<T> chaining so synchronous InMemoryDisputeRepository callers remain synchronous while async PgDisputeRepository callers return Promises.
2. Async Route Handlers & App Mounting (src/routes/billing/disputes.ts, src/app.ts):
   • Updated /api/billing/disputes route handlers to await DisputeService operations.
   • Mounted /api/billing/disputes in src/app.ts backed by new PgDisputeRepository(pool) in non-test environments (falling back to InMemoryDisputeRepository in unit test mode when not overridden).
3. Standard SQL Migration (migrations/0019_disputes.sql):
   • Normalized DDL identifiers and added resolution_notes on disputes and notes on dispute_events so the migration applies cleanly in both PostgreSQL (pg / pg-mem) and SQLite.

Verification

• Added pg-mem integration tests in src/routes/billing/disputes.test.ts covering persistence across fresh DisputeService instances, audit trail creation/status updates, and 409 Conflict on duplicate open disputes.
• Verified with npx jest src/routes/billing/disputes.test.ts (44/44 passed).

Payout Wallet (Base Mainnet USDC)

0x3806cd592a508d1992Ed007C2F53de251C2E7452