feat: ML tag suggestions, character/fandom integrity, underscores, modal polish

Consolidated merge of feat/tag-suggestions branch. Original 64-commit history
was lost to git-object corruption in a Nextcloud-synced checkout; this single
commit captures the equivalent diff.

Includes:
- pgvector-backed tag suggestion infra (WD14 + SigLIP centroids, ml-worker
  container, Celery tasks, suggestion service, accept/reject endpoints + modal
  UI with green/red chip buttons)
- Character/fandom integrity: title-case normalization on every write path,
  fandom-id backfill, maintenance task + settings button, migrations g26041901
  + h26041901 to canonicalize legacy rows with case-only duplicate merging
- Tag-underscores + modal polish: WD14 name canonicalization at emit + accept
  + add/bulk-add paths, migration i26041901 for legacy-row rename-or-merge
  across character/fandom/NULL kinds, suggestion-accept refresh parity via
  awaited loadTags, persistent chip tint

app/templates/_gallery_modal.html

@@ -47,6 +47,11 @@
           </div>
         </div>
 
+        <!-- SECTION: Provenance (system-generated tags: post; future: source, archive) -->
+        <div class="modal-sidebar-section modal-provenance-section" id="modalProvenanceSection" style="display: none;">
+          <div id="modalProvenanceList" class="provenance-list"></div>
+        </div>
+
         <!-- SECTION: Tags -->
         <div class="modal-sidebar-section modal-tags-section">
           <div id="modalTagList" class="tags"></div>
@@ -60,6 +65,15 @@
           <span id="tagActionFeedback" class="tag-feedback tag-feedback-inline"></span>
         </div>
 
+        <!-- SECTION: Suggestions (ML-backed, modal-only) -->
+        <div class="modal-sidebar-section modal-suggestions-section" id="modalSuggestionsSection" style="display: none;">
+          <div class="section-header">
+            <span class="section-title">Suggestions</span>
+            <button type="button" class="suggestions-refresh" id="suggestionsRefreshBtn" aria-label="Refresh suggestions">↻</button>
+          </div>
+          <div id="modalSuggestionsList" class="suggestions-list"></div>
+        </div>
+
       </div>
     </div>
   </div>