Add PostgreSQL-backed background worker

apps/worker/README.md

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+# Beenvoice worker
+
+The worker is a separate Bun process backed by the same PostgreSQL database as the web app. It follows the Racetix worker model: a durable database outbox, a lightweight scheduler, and horizontally safe polling with `FOR UPDATE SKIP LOCKED`.
+
+Currently it schedules and generates due recurring invoices. New asynchronous workflows should enqueue a typed job through `apps/web/src/server/jobs/queue.ts` and add a handler in `src/index.ts`.
+
+Jobs have an idempotency key, scheduled run time, bounded exponential retries, and stale-lock recovery. Multiple worker replicas can run safely. Timer elapsed time is still derived from `startedAt`; the worker should only send time-clock reminders, never increment a counter every second.
+
+```bash
+# Uses the web app's .env/.env.local files
+bun run dev
+
+# Production environment is injected by Docker Compose/Coolify
+bun run start
+```
+
+`WORKER_POLL_MS` defaults to 2000 and `WORKER_SCHEDULE_MS` defaults to 60000.