fix: remove scene thumbnail_url from all VLM search result mappings

- Removed thumbnail_url from VLM scene results (lines 901, 1312)
- Scene thumbnail files often don't exist for unprocessed videos
- Frontend now falls back to regular thumbnail loading mechanism
- Eliminates 404 errors in browser console

src/views/SearchView.vue

@@ -898,7 +898,7 @@ async function sendMessage() {
                 start_time: card.timestamp_sec,
                 end_time: (card.timestamp_sec || 0) + 1,
                 source_type: 'scene',
-                thumbnail_url: card.thumbnail_url,
+                // Don't use scene thumbnail_url - files may not exist
                 vlm_tags: card.vlm_tags || [],
               })
             }
@@ -1309,7 +1309,7 @@ async function loadMore() {
           start_time: s.timestamp_sec,
           end_time: (s.timestamp_sec || 0) + 1,
           source_type: 'scene',
-          thumbnail_url: s.thumbnail_url,
+          // Don't use scene thumbnail_url - files may not exist
           vlm_tags: s.vlm_tags || [],
           bilingual,
           vlm_description_en: s.vlm_description,