character-fandom tag association system

Link character tags to fandom tags via fandom_id FK with auto-apply
behavior. Adding character:Saber (Fate) auto-creates fandom:Fate and
applies it to the image. Renaming a fandom cascades to all linked
characters. Includes set-fandom endpoint for retroactive association
and fandom selector dropdown in the tag editor UI.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/models.py

@@ -43,6 +43,8 @@ class Tag(db.Model):
     id = db.Column(db.Integer, primary_key=True)
     name = db.Column(db.String(255), unique=True, nullable=False)
     kind = db.Column(db.String(64), nullable=True)
+    fandom_id = db.Column(db.Integer, db.ForeignKey("tag.id", ondelete="SET NULL"), nullable=True)
+    fandom = db.relationship("Tag", remote_side="Tag.id", foreign_keys=[fandom_id])
     images = db.relationship(
         "ImageRecord",
         secondary=image_tags,