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

package-lock.json

@@ -34,6 +34,7 @@
         "rxjs": "^7.8.2",
         "tailwindcss": "latest",
         "youtube-dl-exec": "^3.0.0",
+        "youtubei.js": "^18.1.0",
         "zone.js": "~0.15.1"
       },
       "devDependencies": {
@@ -965,6 +966,12 @@
         "node": ">=6.9.0"
       }
     },
+    "node_modules/@bufbuild/protobuf": {
+      "version": "2.15.0",
+      "resolved": "https://registry.npmjs.org/@bufbuild/protobuf/-/protobuf-2.15.0.tgz",
+      "integrity": "sha512-DAheWUkVr/SJTWCc+lg9dhY0eN4SaWlf4+bG1KzHeXbnqt0AfB/NX0Z+VunGlM1ki1B4zVvye27MpKh/svySUA==",
+      "license": "(Apache-2.0 AND BSD-3-Clause)"
+    },
     "node_modules/@cspotcode/source-map-support": {
       "version": "0.8.1",
       "resolved": "https://registry.npmjs.org/@cspotcode/source-map-support/-/source-map-support-0.8.1.tgz",
@@ -5672,6 +5679,12 @@
         }
       }
     },
+    "node_modules/fflate": {
+      "version": "0.8.3",
+      "resolved": "https://registry.npmjs.org/fflate/-/fflate-0.8.3.tgz",
+      "integrity": "sha512-tbZNuJrLwGUp3zshBtdy4W+ORxZuIh8a5ilyIEQDC5rY1f3U20JMry0Ll3WBzU58EZKsEuJFXhb5gwv8CsPvgA==",
+      "license": "MIT"
+    },
     "node_modules/ffmpeg-static": {
       "version": "5.2.0",
       "resolved": "https://registry.npmjs.org/ffmpeg-static/-/ffmpeg-static-5.2.0.tgz",
@@ -6943,6 +6956,15 @@
       "integrity": "sha512-abv/qOcuPfk3URPfDzmZU1LKmuw8kT+0nIHvKrKgFrwifol/doWcdA4ZqsWQ8ENrFKkd67Mfpo/LovbIUsbt3w==",
       "license": "MIT"
     },
+    "node_modules/meriyah": {
+      "version": "7.3.3",
+      "resolved": "https://registry.npmjs.org/meriyah/-/meriyah-7.3.3.tgz",
+      "integrity": "sha512-uE5cnoNj+UYhoMdZDuymCzr5TzEuE3ZF2C4kn6Z76rLhkwnvZ/+6dUZnM41T3fifyuhW0+n8vK7eq6DKJxUfPA==",
+      "license": "ISC",
+      "engines": {
+        "node": ">=20.0.0"
+      }
+    },
     "node_modules/methods": {
       "version": "1.1.2",
       "resolved": "https://registry.npmjs.org/methods/-/methods-1.1.2.tgz",
@@ -9992,6 +10014,20 @@
         "node": ">= 18"
       }
     },
+    "node_modules/youtubei.js": {
+      "version": "18.1.0",
+      "resolved": "https://registry.npmjs.org/youtubei.js/-/youtubei.js-18.1.0.tgz",
+      "integrity": "sha512-4Goo3ZeO6/kE8Mq67eC8Z9K1rijeqRwyKxf/YM4lqf8bB4r/s8JC0V3YFx7+l/AJR3LnAsulTsiIj+AFe0cduA==",
+      "funding": [
+        "https://github.com/sponsors/LuanRT"
+      ],
+      "license": "MIT",
+      "dependencies": {
+        "@bufbuild/protobuf": "^2.0.0",
+        "fflate": "^0.8.2",
+        "meriyah": "^7.3.1"
+      }
+    },
     "node_modules/zod": {
       "version": "3.25.76",
       "resolved": "https://registry.npmjs.org/zod/-/zod-3.25.76.tgz",