refactor(main): gallery routes use ?tag_id=int

Drops the ?tag=<name> lookup. URLs become unambiguous (distinct
same-name characters each have their own id-keyed URL). Eager-loads
Tag.fandom for the active-tag header rendering.

Applied to all four gallery routes: /gallery, /api/gallery/scroll,
/api/gallery/timeline, /api/gallery/jump — they all shared the same
name-keyed filter pattern. (Plan named only the first two; extended
to all four for behavioral consistency so partial JS migration to
tag_id doesn't silently drop filter on the sub-APIs.)

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

app/main.py

@@ -113,17 +113,17 @@ def gallery():
     from app.models import PostMetadata
 
     initial_limit = 30
-    tag_name = request.args.get('tag')
+    tag_id = request.args.get('tag_id', type=int)
+    active_tag_obj = (
+        Tag.query.options(joinedload(Tag.fandom)).get(tag_id)
+        if tag_id else None
+    )
 
     # Base query + eager-load tags
     q = ImageRecord.query.options(joinedload(ImageRecord.tags))
 
-    # Optional tag filter
-    active_tag_obj = None
-    if tag_name:
-        active_tag_obj = Tag.query.filter_by(name=tag_name).first()
-        if active_tag_obj:
-            q = q.join(ImageRecord.tags).filter(Tag.name == tag_name)
+    if active_tag_obj:
+        q = q.join(ImageRecord.tags).filter(Tag.id == active_tag_obj.id)
 
     # Fetch initial batch ordered by date descending
     images = q.order_by(
@@ -169,14 +169,14 @@ def gallery():
         page_count = SeriesPage.query.filter_by(series_tag_id=active_tag_obj.id).count()
         series_info = {
             'tag_id': active_tag_obj.id,
-            'name': active_tag_obj.name.split(':', 1)[1] if ':' in active_tag_obj.name else active_tag_obj.name,
+            'name': active_tag_obj.name,
             'page_count': page_count,
         }
 
     return render_template(
         'gallery.html',
         grouped_images=grouped_images,
-        active_tag=tag_name,
+        active_tag=active_tag_obj.display_name if active_tag_obj else None,
         active_tag_obj=active_tag_obj,
         initial_cursor=initial_cursor,
         has_more=has_more,
@@ -264,14 +264,14 @@ def gallery_scroll():
     """
     cursor_str = request.args.get('cursor')
     limit = request.args.get('limit', 30, type=int)
-    tag_name = request.args.get('tag')
+    tag_id = request.args.get('tag_id', type=int)
 
     # Build base query
     q = ImageRecord.query.options(joinedload(ImageRecord.tags))
 
     # Optional tag filter
-    if tag_name:
-        q = q.join(ImageRecord.tags).filter(Tag.name == tag_name)
+    if tag_id:
+        q = q.join(ImageRecord.tags).filter(Tag.id == tag_id)
 
     # Apply cursor filter (fetch images older than cursor)
     if cursor_str:
@@ -330,7 +330,7 @@ def gallery_timeline():
 
     Returns list of {key, label, count, year} ordered newest to oldest.
     """
-    tag_name = request.args.get('tag')
+    tag_id = request.args.get('tag_id', type=int)
 
     # Use SQL to aggregate by year-month
     date_expr = func.coalesce(ImageRecord.taken_at, ImageRecord.imported_at)
@@ -341,8 +341,8 @@ def gallery_timeline():
         func.count(ImageRecord.id).label('count')
     )
 
-    if tag_name:
-        q = q.join(ImageRecord.tags).filter(Tag.name == tag_name)
+    if tag_id:
+        q = q.join(ImageRecord.tags).filter(Tag.id == tag_id)
 
     q = q.group_by(
         extract('year', date_expr),
@@ -391,7 +391,7 @@ def gallery_jump():
     """
     year_month = request.args.get('year_month', '')
     limit = request.args.get('limit', 30, type=int)
-    tag_name = request.args.get('tag')
+    tag_id = request.args.get('tag_id', type=int)
 
     if not year_month:
         return jsonify(error='year_month is required'), 400
@@ -410,8 +410,8 @@ def gallery_jump():
     # Build query for images in that month and earlier
     q = ImageRecord.query.options(joinedload(ImageRecord.tags))
 
-    if tag_name:
-        q = q.join(ImageRecord.tags).filter(Tag.name == tag_name)
+    if tag_id:
+        q = q.join(ImageRecord.tags).filter(Tag.id == tag_id)
 
     # Get images from start of target month onwards (but ordered descending)
     date_expr = func.coalesce(ImageRecord.taken_at, ImageRecord.imported_at)