fix: commit migration role assumption

Assistant: codex
Assistant-Model: gpt-5.6-sol
Assistant-Session: 01a02b22-9638-76d2-bbff-b7ea1770b118

migrations/env.py

@@ -6,7 +6,7 @@ from alembic import context
 from sqlalchemy import engine_from_config, pool
 
 from sbom_nexus.config import database_target, migration_role
-from sbom_nexus.database import database_url, metadata
+from sbom_nexus.database import assume_migration_role, database_url, metadata
 
 config = context.config
 
@@ -45,7 +45,7 @@ def run_migrations_online() -> None:
         if role and connection.dialect.name != "postgresql":
             raise RuntimeError("SBOM_NEXUS_MIGRATION_ROLE requires PostgreSQL")
         if role:
-            connection.exec_driver_sql(f'SET ROLE "{role}"')
+            assume_migration_role(connection, role)
         context.configure(
             connection=connection,
             target_metadata=target_metadata,