Stabilize Gemini visual fallback handling

TODO.md

@@ -255,6 +255,20 @@
   - backend debug broadcasts
 
 ## Recent Change Log
+- Date: `2026-03-16`
+- What changed:
+  - Stabilized the Gemini visual-review path after widened search budgets caused full-batch “no candidate thumbnails or preview frames” failures.
+  - Google Video enrichment now always prefers the canonical YouTube `ytimg` thumbnail instead of keeping a potentially broken search-engine thumbnail.
+  - Gemini visual fetch now preserves the last concrete fetch error, retries with derived YouTube thumbnails when possible, and only falls back to the generic no-visual message after all image/frame paths fail.
+  - User-facing fallback warning text for the all-batches-no-visual case is now softened so ranked results can still be shown without surfacing the raw internal Gemini error string in the UI.
+- Why it changed:
+  - The latest deployed build widened search enough that top-ranked candidates sometimes carried metadata without any fetchable image bytes, causing Gemini review to fail for every batch and surfacing an alarming warning even though ranked fallback results still existed.
+- How it was verified:
+  - `go test ./...`
+  - `bash scripts/selftest.sh`
+- What is still risky or incomplete:
+  - This improves YouTube-backed candidates immediately, but Envato/Artgrid thumbnails can still be inaccessible in some provider-side cases, so Gemini can still fall back to ranked results when source media is locked down.
+
 - Date: `2026-03-16`
 - What changed:
   - Hid the expanded query-variant chip list from the search UI while leaving backend/debug query visibility intact.