Improve embed handling and Gemini verdict flow

TODO.md

@@ -341,6 +341,21 @@
 - [ ] full browser-level validation was not fully reproducible in this environment
 
 ## Recent Change Log
+- Date: `2026-03-16`
+- What changed:
+  - Google Video embed URL now uses `youtube-nocookie` with explicit `origin` to reduce player load failures.
+  - Gemini Vision prompt now forces a `Yes` or `No` verdict per candidate and only `Yes` candidates are merged into the final result set.
+  - Candidate visual fetch now prefers Envato / Artgrid preview video frames before thumbnails and sends a `Referer` header for thumbnail fetches.
+  - Envato / Artgrid enrichment now retries preview extraction once after a short delay when the first fetch still lacks a usable preview URL.
+- Why it changed:
+  - The user reported YouTube player error `153`, too many fallback-style results, and source pages that appear to need additional loading time before preview URLs become visible.
+- How it was verified:
+  - log review from `ai-media-hub-2026-03-16T05-05-58-704Z.log`
+  - `go test ./...`
+- What is still risky or incomplete:
+  - If a YouTube video has embedding disabled by the uploader, no embed URL variant will fully bypass that restriction.
+  - Delay-and-retry HTML fetching cannot execute JavaScript, so pages that only expose preview URLs after full client-side rendering may still need a real browser-based fetch path later.
+
 - Date: `2026-03-16`
 - What changed:
   - Reduced the heaviest search-stage caps slightly: fewer query variants per request, smaller per-source result caps, lower enrichment scope, and a bounded Gemini candidate set.