fix(migration049): use COMMIT/BEGIN instead of execution_options AUTOCOMMIT

asyncpg+alembic keeps a transaction open via begin_transaction(); calling
execution_options(isolation_level=AUTOCOMMIT) on an already-begun connection
raises InvalidRequestError. Replace with explicit COMMIT before ALTER TYPE
and BEGIN before the backfill UPDATE.

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

backend/alembic/versions/049_role_hierarchy.py

@@ -18,15 +18,20 @@ depends_on = None
 
 
 def upgrade() -> None:
-    # PostgreSQL requires ALTER TYPE ADD VALUE to be committed in its own transaction
-    # before the new value can be used.  Execute each ADD VALUE with AUTOCOMMIT.
+    # PostgreSQL requires ALTER TYPE ADD VALUE to be committed before the new value
+    # can be referenced in the same session.  With asyncpg + alembic the connection
+    # already has an open transaction (begin_transaction), so we cannot change
+    # isolation level via execution_options.  Instead we manually COMMIT the outer
+    # transaction, run the DDL, then open a new transaction for the DML backfill.
     conn = op.get_bind()
 
-    with conn.execution_options(isolation_level="AUTOCOMMIT"):
+    # Commit the alembic-managed transaction so ALTER TYPE runs outside a tx block
+    conn.execute(sa.text("COMMIT"))
     conn.execute(sa.text("ALTER TYPE userrole ADD VALUE IF NOT EXISTS 'global_admin'"))
     conn.execute(sa.text("ALTER TYPE userrole ADD VALUE IF NOT EXISTS 'tenant_admin'"))
 
-    # Now in a normal transaction: backfill existing 'admin' → 'global_admin'
+    # Start a new transaction for the backfill DML
+    conn.execute(sa.text("BEGIN"))
     conn.execute(
         sa.text("UPDATE users SET role = 'global_admin'::userrole WHERE role = 'admin'::userrole")
     )