Feat/idempotency cleanup

[Johnsource-hub]

#closes
#803

New files:

• db/014_idempotency_cleanup_index.sql — Migration adding idx_idempotency_keys_created_at index
• src/services/idempotencyCleanup.ts — Cleanup job with IdempotencyCleanupDriver abstraction, deleteExpiredIdempotencyKeys(), and startIdempotencyCleanupJob() (hourly setInterval, skips when NODE_ENV=test, logs deleted count at debug, increments Prometheus counter)
• tests/services/idempotencyCleanup.test.ts — 3 tests: deletes only expired rows, keeps recent rows, skips job in test env
  Modified files:
• src/middleware/metrics.ts:17-21 — Added idempotencyKeysDeletedTotal Prometheus counter
• src/middleware/idempotency.ts:45-49 — Added created_at index creation in initializeDatabase()
• src/middleware/idempotency.ts:155-161 — Exported cleanupDriver implementing IdempotencyCleanupDriver
• src/index.ts:13-14 — Imported cleanupDriver and startIdempotencyCleanupJob; wired up on line 147
• mocks/better-sqlite3.js — Updated mock's DELETE FROM IDEMPOTENCY_KEYS handler to support created_at-based deletion