fix: add tombstone migration for reverted calendar subscriptions

migrations/versions/0020_calendar_subscriptions.py

@@ -0,0 +1,28 @@
+"""tombstone for reverted calendar subscriptions
+
+Revision ID: 0020_calendar_subscriptions
+Revises: 0019_backup_imports
+
+The calendar subscription feature (backend/calendar.py, backend/calendar_router.py)
+was reverted from main, but this revision id was already applied to the production
+database. Keeping an empty migration under the same revision id lets
+`alembic upgrade head` succeed on databases stamped at this revision.
+"""
+
+from alembic import op  # noqa: F401
+
+revision = "0020_calendar_subscriptions"
+down_revision = "0019_backup_imports"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    # Intentionally empty: the feature was reverted. The leftover
+    # calendar_subscriptions table on already-migrated databases is harmless
+    # and intentionally left in place.
+    pass
+
+
+def downgrade() -> None:
+    pass