feat(tags): edit a character tag's fandom (backend)

No way existed to change which fandom a character tag belongs to after
creation — PATCH /tags/<id> only renamed.

- TagService.set_fandom(tag_id, fandom_id, merge=False): set / change /
  clear (fandom_id=None) a character's fandom, with the same validation as
  find_or_create. On a name collision in the target fandom it raises
  TagMergeConflict (→ 409, same shape as rename); merge=True resolves it by
  merging this tag INTO the existing character.
- Extract _do_merge(source, target) from merge() so set_fandom can perform
  the deliberate CROSS-fandom merge the public merge() validation forbids.
- PATCH /tags/<id> now accepts optional fandom_id (+ merge flag) alongside
  name, and returns fandom_id.

Tests: set/change/clear, non-character + bad-ref rejection, collision
raises, merge resolves; API set/clear + collision→merge.

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

backend/app/api/tags.py

@@ -195,14 +195,27 @@ async def remove_tag_from_image(image_id: int, tag_id: int):
 
 
 @tags_bp.route("/tags/<int:tag_id>", methods=["PATCH"])
-async def rename_tag(tag_id: int):
-    body = await request.get_json()
-    if not body or "name" not in body:
-        return jsonify({"error": "name required"}), 400
+async def update_tag(tag_id: int):
+    """Rename and/or re-fandom a tag. Body may carry `name` and/or
+    `fandom_id` (a fandom tag id, or null to clear — character tags only).
+    `merge: true` resolves a collision by merging into the existing tag.
+    """
+    body = await request.get_json() or {}
+    has_name = "name" in body
+    has_fandom = "fandom_id" in body
+    if not has_name and not has_fandom:
+        return jsonify({"error": "name or fandom_id required"}), 400
+    do_merge = bool(body.get("merge"))
     async with get_session() as session:
         svc = TagService(session)
         try:
-            tag = await svc.rename(tag_id, body["name"])
+            tag = None
+            if has_name:
+                tag = await svc.rename(tag_id, body["name"])
+            if has_fandom:
+                tag = await svc.set_fandom(
+                    tag_id, body["fandom_id"], merge=do_merge
+                )
         except TagMergeConflict as exc:
             return jsonify(
                 {
@@ -219,7 +232,12 @@ async def rename_tag(tag_id: int):
             return jsonify({"error": str(exc)}), 400
         await session.commit()
         return jsonify(
-            {"id": tag.id, "name": tag.name, "kind": tag.kind.value}
+            {
+                "id": tag.id,
+                "name": tag.name,
+                "kind": tag.kind.value,
+                "fandom_id": tag.fandom_id,
+            }
         )