chore(prod): startup migrations, readiness checks and backend healthcheck

- add backend entrypoint that can run alembic upgrade head on startup
- add RUN_MIGRATIONS_ON_STARTUP setting and compose wiring
- add /health/live and /health/ready endpoints with db+redis checks
- add backend container healthcheck against readiness endpoint
- document readiness and startup migration behavior

README.md

@@ -38,12 +38,17 @@ Run full stack (web + api + worker + postgres + redis + minio + mailpit):
 1. cp .env.docker.example .env
 2. edit `.env` (`SECRET_KEY`, passwords, domain, `S3_PUBLIC_ENDPOINT_URL`)
 3. docker compose up -d --build
-2. Open:
+4. check backend readiness:
+   - `http://localhost:8000/health/live`
+   - `http://localhost:8000/health/ready`
+5. Open:
    - Web: http://localhost
    - API docs: http://localhost:8000/docs
    - Mailpit UI: http://localhost:8025
    - MinIO console: http://localhost:9001
 
+`RUN_MIGRATIONS_ON_STARTUP=true` (default in compose) runs `alembic upgrade head` before backend start.
+
 ### Production Mode
 
 Use production override to close internal ports (postgres/redis/minio/mailpit/backend):