feat(youtube): InnerTube-first search, transcripts and watch-next related (Steps 15-18)

- InnerTube layer via pinned youtubei.js 18.1.0 (no quota, no key):
  search with merged continuations (unlimited pages), watch-next
  related with LockupView mapping, caption-track discovery
- 3-layer dispatcher (YT_SEARCH_MODE, default innertube-first):
  innertube -> yt-dlp scrape -> official API, graceful errors.yt
- Robust yt-dlp binary resolution (YT_DLP_PATH > PATH > bundled)
  with systematic API fallback (fixes spawn ENOENT in UI)
- Transcript: InnerTube caption discovery (YT_TRANSCRIPT_SOURCE),
  reusing pickTrack/orderedTracks/parseTrackText; yt-dlp fallback kept
- Watch: sidebar uses real watch-next related[] (/api/details),
  title-search fallback for other providers
- Cache: memory LRU + SQLite (youtube_search_cache, youtube_metrics),
  never persist empty pages; /healthz observability; /api/trending
- Includes pending Step 15/16 leftovers in same files (suggest,
  test scripts); unrelated provider adapters left uncommitted

docker-compose/.env.example

@@ -15,6 +15,23 @@ TWITCH_CLIENT_SECRET=votre_client_secret_twitch_ici
 # Configuration du cache (en millisecondes)
 YT_CACHE_TTL_MS=3600000  # 1 heure par défaut
 
+# --- YouTube hybride : InnerTube (sans clé) > scrape yt-dlp > API officielle ---
+# Mode : innertube-first (défaut, 0 quota, pagination illimitée) | innertube-only
+#      | scrape-first | api-first | scrape-only | api-only
+YT_SEARCH_MODE=innertube-first
+YT_INNERTUBE_GL=FR
+YT_INNERTUBE_HL=fr
+# Sous-titres YouTube : innertube-first (défaut) | ytdlp-only | innertube-only
+YT_TRANSCRIPT_SOURCE=innertube-first
+# Cache scrape/pagination (30 min) + timeout yt-dlp
+YT_SCRAPE_TTL_MS=1800000
+YT_DLP_TIMEOUT_MS=20000
+# Anti-ban (optionnel) : binaire explicite, cookies exportés, PO-Token, proxy sortant
+# YT_DLP_PATH=/usr/local/bin/yt-dlp
+# YT_COOKIES_FILE=/cookies/youtube.txt
+# YT_PO_TOKEN=
+# YT_EGRESS_PROXY=http://proxy:8080
+
 # Téléchargements : providers autorisés (liste CSV) + concurrence max par user
 DOWNLOAD_PROVIDERS=youtube,dailymotion,twitch,peertube,odysee,rumble
 DOWNLOAD_MAX_CONCURRENT=2