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>

migrations/versions/f26021101_add_fandom_id_to_tag.py

@@ -0,0 +1,30 @@
+"""Add fandom_id to tag for character-fandom association
+
+Revision ID: f26021101
+Revises: e26012202
+Create Date: 2026-02-11
+
+"""
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision = 'f26021101'
+down_revision = 'e26012202'
+branch_labels = None
+depends_on = None
+
+
+def upgrade():
+    op.add_column('tag', sa.Column('fandom_id', sa.Integer(), nullable=True))
+    op.create_foreign_key(
+        'fk_tag_fandom_id', 'tag', 'tag',
+        ['fandom_id'], ['id'],
+        ondelete='SET NULL'
+    )
+
+
+def downgrade():
+    op.drop_constraint('fk_tag_fandom_id', 'tag', type_='foreignkey')
+    op.drop_column('tag', 'fandom_id')