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

.github/workflows/ci.yml

@@ -42,3 +42,15 @@ jobs:
 
       - name: E2E scenarios (search UX)
         run: npm run test:search-e2e
+
+      - name: Suggest typeahead (Step 15)
+        run: npm run test:suggest
+
+      - name: Transcripts (Step 16)
+        run: npm run test:transcript
+
+      - name: YouTube scrape-first (Step 17, offline)
+        run: npm run test:ytscrape
+
+      - name: YouTube InnerTube (Step 18, offline)
+        run: npm run test:ytinnertube