Implement Redis-backed WebSocket Pub/Sub helpers

[emiliano-go]

Implement Redis-backed WebSocket Pub/Sub helpers (Issue #24)

5 commits:

feat: add PubSubManager for Redis Pub/Sub messaging
pubsub.py, __init__.py, deps.py, telemetry.py

PubSubManager with subscribe/listen/publish/close. FastAPI DI via get_pubsub_manager (sub-dependency on Depends(get_async_redis)). record_pubsub_publish telemetry counter.

test: add 25 PubSub unit tests
tests/unit/test_pubsub.py

Core lifecycle, 13 edge cases, memory leak via weakref, DI integration.

test: add PubSub integration tests
tests/integration/test_pubsub.py

Cross-connection bridge, subscriber count, unsub isolation (requires real Redis).

docs: add PubSub user guide and API reference
docs/guide/pubsub.md, docs/api/reference.md, mkdocs.yml

Guide with WebSocket examples, connection pool limitation note, DI override chains.

examples: add PubSub WebSocket chat and notify endpoint
examples/main.py

/notify HTTP publish and /ws/chat/{room} WebSocket broadcast loop.

Design decisions

• get_pubsub_manager uses Depends(get_async_redis) instead of calling get_async_redis directly, so callers can override the Redis client without re-registering the PubSub provider.
• listen() filters system events and yields only message-type events.
• Each subscribe() call consumes one dedicated pool connection (inherent Redis Pub/Sub limitation), documented in the guide.
• No new middleware or lifespan changes needed; Pub/Sub reuses the existing shared pool.

Closes #24

[emiliano-go]

I just noticed PR #27 after opening this one (started working yesterday at night, too tired). The two PRs target the same feature but go about it differently.
I've incorporated the HTTPConnection approach from PR #27 into get_async_redis so it works in both HTTP and WebSocket endpoints directly. Credit to @massooti for that improvement.