refactor(tags): shared tag_query for fandom self-join + serialization (DRY sweep)

The fandom self-join (resolve a character's fandom NAME via Tag.fandom_id->Tag)
and the {id,name,kind,fandom_id,fandom_name} dict were hand-written in
TagService.autocomplete/.list_for_image, GalleryService.get_image_with_tags and
the api/tags handlers — the last few grown by this session's fandom-on-chip
feature. Consolidate to services/tag_query: fandom_join_alias() + tag_columns()
build the select; serialize_tag(row) builds the dict. Now a new tag field is
added in one place.

Over-DRY guard: TagDirectoryService selects the full Tag ORM + an image-count
aggregate (a different select shape) — left as its own variant. §8b: the
fandom_lookup alias lives only in tag_query; gallery + both api/tags handlers
serialize via serialize_tag. Test: serialize_tag handles enum + string kind.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/app/api/tags.py

@@ -11,6 +11,7 @@ from ..services.bulk_tag_service import BulkTagService
 from ..services.series_match_service import SeriesMatchService
 from ..services.series_service import SeriesError, SeriesService
 from ..services.tag_directory_service import TagDirectoryService
+from ..services.tag_query import serialize_tag
 from ..services.tag_service import (
     TagMergeConflict,
     TagService,
@@ -72,17 +73,7 @@ async def autocomplete():
         hits = await svc.autocomplete(q, kind=kind, limit=limit)
 
     return jsonify(
-        [
-            {
-                "id": h.id,
-                "name": h.name,
-                "kind": h.kind,
-                "fandom_id": h.fandom_id,
-                "fandom_name": h.fandom_name,
-                "image_count": h.image_count,
-            }
-            for h in hits
-        ]
+        [{**serialize_tag(h), "image_count": h.image_count} for h in hits]
     )
 
 
@@ -165,18 +156,7 @@ async def list_tags_for_image(image_id: int):
     async with get_session() as session:
         svc = TagService(session)
         tags = await svc.list_for_image(image_id)
-    return jsonify(
-        [
-            {
-                "id": t.id,
-                "name": t.name,
-                "kind": t.kind.value,
-                "fandom_id": t.fandom_id,
-                "fandom_name": t.fandom_name,
-            }
-            for t in tags
-        ]
-    )
+    return jsonify([serialize_tag(t) for t in tags])
 
 
 @tags_bp.route("/images/<int:image_id>/tags", methods=["POST"])