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,
+            }
         )
 
 

backend/app/services/tag_service.py

@@ -280,6 +280,69 @@ class TagService:
         await self.session.flush()
         return tag
 
+    async def set_fandom(
+        self, tag_id: int, fandom_id: int | None, *, merge: bool = False
+    ) -> Tag:
+        """Set / change / clear a character tag's fandom.
+
+        Raises TagValidationError unless the tag is a character and fandom_id
+        (when given) references a fandom tag. If the change would collide with
+        an existing character of the same name in the TARGET fandom, raises
+        TagMergeConflict (the API turns that into a 409 merge hint) — unless
+        merge=True, in which case this tag is merged INTO that existing
+        character (a deliberate cross-fandom merge) and the surviving target
+        is returned. Passing fandom_id=None clears the fandom.
+        """
+        tag = await self.session.get(Tag, tag_id)
+        if tag is None:
+            raise TagValidationError(f"Tag {tag_id} not found")
+        if tag.kind != TagKind.character:
+            raise TagValidationError("Only character tags can have a fandom")
+        if fandom_id is not None:
+            fandom = await self.session.get(Tag, fandom_id)
+            if fandom is None or fandom.kind != TagKind.fandom:
+                raise TagValidationError(
+                    f"fandom_id {fandom_id} does not reference a fandom tag"
+                )
+        if fandom_id == tag.fandom_id:
+            return tag
+
+        # Collision: another character with the same name already lives in the
+        # target fandom. Mirrors rename's (name, kind, fandom_id) uniqueness.
+        clash_stmt = (
+            select(Tag)
+            .where(Tag.name == tag.name)
+            .where(Tag.kind == TagKind.character)
+            .where(
+                Tag.fandom_id.is_(None)
+                if fandom_id is None
+                else Tag.fandom_id == fandom_id
+            )
+            .where(Tag.id != tag_id)
+        )
+        clash = (await self.session.execute(clash_stmt)).scalar_one_or_none()
+        if clash is not None:
+            if not merge:
+                source_image_count = await self.session.scalar(
+                    select(func.count())
+                    .select_from(image_tag)
+                    .where(image_tag.c.tag_id == tag_id)
+                )
+                will_alias = await self._keep_as_alias(tag_id)
+                raise TagMergeConflict(
+                    f"A character named {tag.name!r} already exists in that fandom",
+                    target_id=clash.id,
+                    target_name=clash.name,
+                    source_image_count=int(source_image_count or 0),
+                    will_alias=will_alias,
+                )
+            await self._do_merge(tag, clash)
+            return clash
+
+        tag.fandom_id = fandom_id
+        await self.session.flush()
+        return tag
+
     async def merge(self, source_id: int, target_id: int) -> MergeResult:
         """Transactionally repoint every FK from source→target, optionally
         keep source's name as a tagger alias, delete source. Atomic: any
@@ -298,7 +361,14 @@ class TagService:
             raise TagValidationError(
                 "Tags must be the same kind and fandom to merge"
             )
+        return await self._do_merge(source, target)
 
+    async def _do_merge(self, source: Tag, target: Tag) -> MergeResult:
+        """Repoint every FK source→target, optionally keep source's name as a
+        tagger alias, delete source. NO kind/fandom validation — callers that
+        need it (public merge()) validate first; set_fandom's collision
+        resolution calls this directly for a deliberate CROSS-fandom merge."""
+        source_id, target_id = source.id, target.id
         keep_as_alias = await self._keep_as_alias(source_id)
         source_name = source.name
         source_kind = source.kind