refactor(main): set_tag_fandom is pure fandom_id update

No rename, no retroactive-backfill-in-second-txn. display_name @property
reflects the change instantly. The auto-apply side effect (attach fandom
tag to images with the character) is preserved but simplified.

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

app/main.py

@@ -1038,60 +1038,35 @@ def get_tag(tag_id):
 def set_tag_fandom(tag_id):
     """
     Set or clear a character tag's fandom association.
-    Updates the tag name to include/remove the (Fandom) suffix.
 
     Form params:
-        fandom_id: ID of an existing fandom tag (optional, clears if absent)
-        fandom_name: Name to find/create a fandom tag (used if fandom_id not given)
+        fandom_id:   ID of an existing fandom tag (preferred).
+        fandom_name: Name to find/create a fandom tag (used if fandom_id
+                     not given). If both omitted, the fandom is cleared.
     """
     tag = Tag.query.get_or_404(tag_id)
-
     if tag.kind != "character":
         return jsonify(ok=False, error="Only character tags can have fandoms"), 400
 
     fandom_id = request.form.get("fandom_id", type=int)
     fandom_name = (request.form.get("fandom_name") or "").strip()
 
-    # Get the character's base name (strip existing fandom suffix) and normalize it.
-    char_part = tag.name.split(":", 1)[1] if ":" in tag.name else tag.name
-    char_base, _ = _parse_character_fandom(char_part)
-    char_base = normalize_display_name(char_base)
-
     if fandom_id:
-        fandom_tag = Tag.query.get(fandom_id)
-        if not fandom_tag or fandom_tag.kind != "fandom":
+        fandom_tag = Tag.query.filter_by(id=fandom_id, kind="fandom").first()
+        if fandom_tag is None:
             return jsonify(ok=False, error="Invalid fandom tag"), 400
-        fandom_display = fandom_tag.name.split(":", 1)[1] if ":" in fandom_tag.name else fandom_tag.name
-    elif fandom_name:
-        # _ensure_fandom_tag normalizes internally.
-        fandom_tag = _ensure_fandom_tag(fandom_name)
-        fandom_display = fandom_tag.name.split(":", 1)[1]
-    else:
-        # Clear fandom
-        tag.fandom_id = None
-        tag.name = f"character:{char_base}"
-        db.session.commit()
-        return jsonify(ok=True, tag={
-            "id": tag.id,
-            "name": tag.name,
-            "kind": tag.kind,
-            "fandom_id": None,
-            "fandom_name": None
-        })
-
-    # Check for name conflict before renaming
-    new_name = f"character:{char_base} ({fandom_display})"
-    conflict = Tag.query.filter(Tag.name == new_name, Tag.id != tag_id).first()
-    if conflict:
-        return jsonify(ok=False, error=f"A tag named '{new_name}' already exists"), 409
-
         tag.fandom_id = fandom_tag.id
-    tag.name = new_name
+    elif fandom_name:
+        fandom_tag = _ensure_fandom_tag(fandom_name)
+        tag.fandom_id = fandom_tag.id
+    else:
+        tag.fandom_id = None
+        fandom_tag = None
+
     db.session.commit()
 
-    # Retroactive backfill: every image already tagged with this character should
-    # also have the fandom tag attached. Runs in a second transaction so a failed
-    # backfill doesn't roll back the rename.
+    # Auto-apply the fandom tag to images already tagged with this character.
+    if fandom_tag is not None:
         try:
             db.session.execute(
                 text("""
@@ -1099,29 +1074,25 @@ def set_tag_fandom(tag_id):
                     SELECT it.image_id, :fandom_id
                     FROM image_tags it
                     WHERE it.tag_id = :char_id
-                  AND NOT EXISTS (
-                      SELECT 1 FROM image_tags it2
-                      WHERE it2.image_id = it.image_id
-                        AND it2.tag_id = :fandom_id
-                  )
+                    ON CONFLICT DO NOTHING
                 """),
-            {'char_id': tag.id, 'fandom_id': fandom_tag.id},
+                {"char_id": tag.id, "fandom_id": fandom_tag.id},
             )
             db.session.commit()
         except Exception as e:
             db.session.rollback()
-        # Rename already committed; log and continue. Sweep button recovers.
             current_app.logger.warning(
-            "set_tag_fandom backfill failed for tag %s -> fandom %s: %s",
+                "set_tag_fandom auto-apply failed for tag %s -> fandom %s: %s",
                 tag.id, fandom_tag.id, e,
             )
 
     return jsonify(ok=True, tag={
         "id": tag.id,
         "name": tag.name,
+        "display_name": tag.display_name,
         "kind": tag.kind,
         "fandom_id": tag.fandom_id,
-        "fandom_name": fandom_display
+        "fandom_name": fandom_tag.name if fandom_tag else None,
     })