feat(jobs): daily cleanup for expired idempotency_keys

[MBombeck]

Summary

Closes Security-Review HIGH-2 from PR #117.

withIdempotency() only purges rows lazily on the next lookup with the same (userId, key, method, path) tuple — most rows never see a retry, so without a sweeper the idempotency_keys table grows unbounded with stale 24h-expired entries.

• New pg-boss queue idempotency-cleanup runs daily at 03:00 Europe/Berlin (parallel to the weekly Sunday data-backup job)
• Bulk-deletes via prisma.idempotencyKey.deleteMany({ where: { expiresAt: { lt: new Date() } } }) — uses the existing idx on expiresAt
• Outcome surfaces as Wide Event annotation idempotency_cleanup_deleted so the run is queryable in Loki
• Cleanup logic extracted to src/lib/jobs/idempotency-cleanup.ts for unit-testable separation from the pg-boss harness

Files

• src/lib/jobs/idempotency-cleanup.ts (new) — exported cleanupExpiredIdempotencyKeys(prisma) returns deleted count
• src/lib/jobs/__tests__/idempotency-cleanup.test.ts (new) — 3 tests: success path, empty table, error propagation
• src/lib/jobs/reminder-worker.ts — register queue, schedule, payload type, handler, worker (+29 LOC, surgical)

Test plan

• pnpm typecheck clean
• pnpm test — 282/282 pass (3 new in idempotency-cleanup.test.ts)
• Verified existing @@index([expiresAt]) on the model — sweep is index-backed, not a full scan
• After deploy: confirm queue is registered and first 03:00 run logs idempotency_cleanup_deleted in Grafana/Loki

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