feat: add supervised background workers

[ItayTheDar]

Summary

• add dependency-injected BackgroundWorker and fixed-delay IntervalWorker APIs
• add WorkerHost supervision with restart policies, capped exponential backoff, status reporting, cooperative shutdown, and forced cancellation after a grace timeout
• start workers on the FastAPI lifespan loop and stop them before provider shutdown hooks
• add standalone WorkerAppFactory and run_workers() support with SIGTERM/SIGINT handling
• document worker deployment semantics and replace the unsafe lifespan-task recipe

Why

Creating long-lived asyncio tasks during the existing synchronous factory bootstrap can bind them to a temporary event loop that is closed immediately afterward. This feature gives background work a first-class runtime that creates tasks on the serving loop, while preserving normal PyNest dependency injection and lifecycle ordering.

Developer impact

Workers are registered as normal module providers and can use constructor injection. HTTP applications require no extra startup code, while dedicated worker processes can run without FastAPI. Recurring jobs can use IntervalWorker; cron/calendar semantics remain intentionally outside core because they require explicit timezone, persistence, misfire, and distributed-locking policies.

Validation

• uv run pytest -q — 201 passed
• Black check on all changed Python files — passed
• uv run --group docs mkdocs build --strict — passed
• git diff --check — passed