Add scheduled invoice delivery

apps/worker/README.md

@@ -2,10 +2,12 @@
 
 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`.
+It generates due recurring invoices and delivers scheduled invoice emails. 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.
 
+Scheduled sends store an absolute UTC instant plus the IANA timezone selected by the client. The worker calls the app's secret-protected internal delivery endpoint through `APP_INTERNAL_URL`; that endpoint passes the job idempotency key to Resend, so a retry cannot send the same invoice twice.
+
 ```bash
 # Uses the web app's .env/.env.local files
 bun run dev
@@ -15,3 +17,4 @@ bun run start
 ```
 
 `WORKER_POLL_MS` defaults to 2000 and `WORKER_SCHEDULE_MS` defaults to 60000.
+`APP_INTERNAL_URL` defaults to `http://app:3000` in Compose, and `CRON_SECRET` authenticates worker requests to the app.