Add merge migration to unify diverged alembic heads

Two branches diverged from e5c3f9a2b1d6 (tag_groups):
- domain_events → generic_containers (g7e5b1c3d4f8)
- tickets → page_configs → market_places → page_tracking (c3d4e5f6a7b8)

This merge migration joins them into a single head so
alembic upgrade head works without specifying a branch.

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

alembic/versions/h8f6c2d4e5a9_merge_heads.py

@@ -0,0 +1,23 @@
+"""merge heads
+
+Revision ID: h8f6c2d4e5a9
+Revises: c3d4e5f6a7b8, g7e5b1c3d4f8
+Create Date: 2026-02-11 00:00:00.000000
+
+"""
+from alembic import op
+import sqlalchemy as sa
+
+# revision identifiers, used by Alembic.
+revision = 'h8f6c2d4e5a9'
+down_revision = ('c3d4e5f6a7b8', 'g7e5b1c3d4f8')
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    pass
+
+
+def downgrade() -> None:
+    pass