feat: add high-level Redis Pub/Sub API for real-time applications

[massooti]

This is belonging to this issue #24:

Without this feature, developers must manually:

• Create and close Redis Pub/Sub connections.
• Handle subscription cleanup when WebSockets disconnect.
• Coordinate concurrent WebSocket reads and Redis messages.
• Reimplement channel prefixes, serialization, cluster handling, and test overrides.
• Avoid an in-memory connection manager that only works inside one process.

With the helper:

Worker A ─┐
Worker B ─┼── Redis channel: room:42 ── broadcast to every connected client
Worker C ─┘

This adds a new project capability: the SDK moves from HTTP caching and raw Redis injection into reusable real-time FastAPI infrastructure. It directly supports chat
rooms, live notifications, job-progress updates, dashboards, and cross-worker broadcasts while reusing the existing Redis pool.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 96.96970% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 98.23%. Comparing base (d98446c) to head (19c7baa).

Files with missing lines	Patch %	Lines
src/redis_fastapi/deps.py	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #27      +/-   ##
==========================================
- Coverage   98.29%   98.23%   -0.06%     
==========================================
  Files           9       10       +1     
  Lines         764      795      +31     
==========================================
+ Hits          751      781      +30     
- Misses         13       14       +1     

Flag	Coverage Δ
unittests	98.23% <96.96%> (-0.06%)	⬇️

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