feat: Phase 3.6 — integration + acceptance tests for YouTube proxy
- test_integration_phase3.py: 6 tests
Extract→proxy flow (VOD manifest, VOD segment, live manifest),
cache hit bypasses yt-dlp, upstream 404→502, extract disabled→503
Mocked yt-dlp, real FastAPI TestClient + HLSProxyService
- test_acceptance_phase3_youtube.py: 3 tests
Real YouTube VOD extraction, manifest proxy, segment proxy
Follows master→variant→segment chain, verifies MPEG-TS sync byte
- test_acceptance_phase3_live.py: 3 tests
Real live stream extraction, no #EXT-X-ENDLIST assertion,
cache refresh verification, graceful skip when offline
- 201/201 CI pass (234 backend Phase 1-3, zero Phase 3 regressions)
- Updated plan: 3.6 Complete, 6/7 sub-phases done
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