feat: auto-discover talk backend

[lan17]

Summary

• add non-secret RTSP camera identity hints so ONVIF-discovered cameras can preserve manufacturer/model metadata for talk backend selection
• keep ONVIF first in auto mode, then select Tapo from persisted identity or a passive local digest endpoint hint after ONVIF is unsupported
• let manual RTSP cameras try passive Tapo discovery and fall back to username admin plus the RTSP password for Tapo credentials
• update ONVIF camera creation payloads, tests, README, and push-to-talk docs

Validation

• make lint
• make typecheck
• uv run pytest tests/homesec/talk/tapo/test_detector.py tests/homesec/talk/tapo/test_client.py tests/homesec/talk/tapo/test_backend.py tests/homesec/talk/tapo/test_tapo_selector.py tests/homesec/rtsp/test_runtime.py -q
• cd ui && pnpm test -- OnvifDiscoveryWizard.test.tsx --run
• make ui-check
• TEST_DB_DSN=postgresql://homesec:homesec@localhost:15433/homesec make check

Notes

• Docker Desktop was started for validation. Port 5432 was already occupied by an agent-control Postgres container, so HomeSec Postgres was run on localhost:15433 for the full check.
• Related Notion epic: https://www.notion.so/362d8336c59f8176af00ce7616cd0841

[codecov]

Codecov Report

❌ Patch coverage is 91.93548% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.19%. Comparing base (0d677d9) to head (7f6bc22).

Files with missing lines	Patch %	Lines
src/homesec/sources/rtsp/core.py	91.30%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #106      +/-   ##
==========================================
+ Coverage   87.16%   87.19%   +0.02%     
==========================================
  Files         151      151              
  Lines       16422    16537     +115     
==========================================
+ Hits        14314    14419     +105     
- Misses       2108     2118      +10     

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