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>

tests/test_tag_query.py

@@ -0,0 +1,29 @@
+"""serialize_tag — the shared canonical tag-dict serializer."""
+
+from types import SimpleNamespace
+
+from backend.app.models import TagKind
+from backend.app.services.tag_query import serialize_tag
+
+
+def test_serialize_tag_with_enum_kind():
+    # ORM/column rows carry kind as a TagKind enum.
+    row = SimpleNamespace(
+        id=1, name="Sasuke Uchiha", kind=TagKind.character,
+        fandom_id=5, fandom_name="Naruto",
+    )
+    assert serialize_tag(row) == {
+        "id": 1, "name": "Sasuke Uchiha", "kind": "character",
+        "fandom_id": 5, "fandom_name": "Naruto",
+    }
+
+
+def test_serialize_tag_with_string_kind_and_no_fandom():
+    # autocomplete hits already store kind as a plain string.
+    row = SimpleNamespace(
+        id=2, name="solo", kind="general", fandom_id=None, fandom_name=None,
+    )
+    assert serialize_tag(row) == {
+        "id": 2, "name": "solo", "kind": "general",
+        "fandom_id": None, "fandom_name": None,
+    }