feat(tags): fandom-less character nudges

Adds a header counter ('⚠ N characters need a fandom') that filters to
?null_fandom=1, plus an inline chip on each qualifying character card.
Backend accepts null_fandom_only in the list query.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

app/templates/_tag_cards.html

@@ -26,6 +26,9 @@
           {%- else -%}#{%- endif -%}
         </span>
         <span class="tag-name" title="{{ tag.name }}">{{ tag.name }}</span>
+        {% if tag.kind == 'character' and tag.fandom_id is none and not show_null_fandom_filter %}
+        <span class="character-no-fandom-chip" title="This character has no fandom — click to assign">⚠ No fandom</span>
+        {% endif %}
         <span class="tag-count">{{ tag.count }}</span>
       </div>
     </a>

app/templates/tags_list.html

@@ -80,6 +80,20 @@
      aria-current="{{ 'page' if active_kind=='user' else 'false' }}"># User</a>
 </div>
 
+{% if null_fandom_character_count > 0 and not show_null_fandom_filter %}
+<a href="{{ url_for('main.tag_list', kind='character', null_fandom=1) }}"
+   class="null-fandom-counter-link">
+  ⚠ {{ null_fandom_character_count }} character{{ 's' if null_fandom_character_count != 1 }} need a fandom
+</a>
+{% endif %}
+
+{% if show_null_fandom_filter %}
+<div class="null-fandom-filter-active">
+  Showing characters without a fandom.
+  <a href="{{ url_for('main.tag_list', kind='character') }}">Clear filter</a>
+</div>
+{% endif %}
+
 {% if tag_data %}
 <div class="tag-grid" id="tagGrid">
   {% include '_tag_cards.html' %}