feat: Phase 3.1 + 3.2 — YouTube config infra and URL extraction
Phase 3.1 — Configuration & Infrastructure:
- Add youtube_proxy_enabled, yt_dlp_timeout, yt_dlp_cache_ttl config fields
- Add yt-dlp and hls.js dependencies
- Create models/youtube.py (request/response schemas)
- Create service stubs (youtube_service, hls_proxy)
- Create router stub and register in main.py
- 11 config tests
Phase 3.2 — YouTube URL Extraction:
- yt-dlp wrapper with async extraction (run_in_executor)
- Format selection: ≤480p video-only + highest-bitrate audio (VOD)
- Combined format fallback: same URL for live streams
- In-memory URL cache: 5min TTL live, 30min VOD
- lru_cache singleton service for cache persistence
- Error handling: DownloadError → 200 with error field
- 18 extract tests, 82/82 total pass (zero regressions)
Real-URL verified: VOD (5bF3tkO5jAA) 24 formats, Live (fN9uYWCjQaw) 6 HLS
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