Implement idempotency key expiry and cleanup job

[Johnsource-hub]

#closes
#794

Acceptance Criteria

• Pool max is respected — requests queue (not crash) when at capacity
• Pool error events are counted in Prometheus (db_pool_error_total)
• DB_POOL_MIN and DB_POOL_MAX documented in .env.example
• SSL modes (true, no-verify, false) work per tests/db/postgresDriverSsl.test.ts
• PgBouncer-compatible SQL (no SET in transaction scope)
• All tests in tests/db/postgresDriverClose.test.ts pass
• validate-env.js CI check passes (all new env vars in .env.example)
• TypeScript type check passes (tsc --noEmit)