character-fandom tag association system

Link character tags to fandom tags via fandom_id FK with auto-apply
behavior. Adding character:Saber (Fate) auto-creates fandom:Fate and
applies it to the image. Renaming a fandom cascades to all linked
characters. Includes set-fandom endpoint for retroactive association
and fandom selector dropdown in the tag editor UI.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/templates/tags_list.html

@@ -135,6 +135,20 @@
         </select>
       </div>
 
+      <div class="form-group" id="fandomGroup" style="display: none;">
+        <label class="form-label" for="editTagFandom">Fandom</label>
+        <div style="position: relative;">
+          <input type="text"
+                 id="editTagFandom"
+                 class="form-input"
+                 placeholder="Type to search fandoms..."
+                 autocomplete="off">
+          <input type="hidden" id="editTagFandomId">
+          <div id="fandomAutocomplete" class="tag-autocomplete" style="display: none;"></div>
+        </div>
+        <small class="form-hint">Select an existing fandom or type a new one. Leave empty to remove fandom.</small>
+      </div>
+
       <div class="form-actions">
         <button type="submit" class="btn primary-btn">Save Changes</button>
         <button type="button" id="tagDeleteBtn" class="btn danger-btn">Delete Tag</button>