Add video reference tracking and display

- Add "Most referenced" sort option to sort by backlink count
- Backend now supports sorting by referenced_by_count field
- Search results now display reference counts as badges:
  - Shows number of backlinks (videos linking to this one)
  - Shows number of internal references (outbound links)
- Reference badges appear alongside transcript source badges

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

static/index.html

@@ -38,11 +38,17 @@
               </div>
             </details>
 
+            <label for="year" style="margin-left: 8px;">Year:</label>
+            <select id="year">
+              <option value="">All Years</option>
+            </select>
+
             <label for="sort" style="margin-left: 8px;">Sort:</label>
             <select id="sort">
               <option value="relevant">Most relevant</option>
               <option value="newer">Newest first</option>
               <option value="older">Oldest first</option>
+              <option value="referenced">Most referenced</option>
             </select>
 
             <label for="size" style="margin-left: 8px;">Size:</label>