Own migrate objects as audit_core_migrate; pin snapshot-read image

SET ROLE during migrate so tables are not owned by a disposable lease.
Current image sha256:aec5575. T02/T03 recorded done after live rotation
and rollout undo. T05 remote run is partial (port-forward dropped).

audit_core/postgres_backend.py

@@ -204,6 +204,13 @@ class PostgresAuditBackend:
         applied: list[str] = []
         try:
             with self.pool.connection() as conn:
+                migrate_role = os.environ.get("AUDIT_CORE_MIGRATE_ROLE", "").strip()
+                if migrate_role:
+                    if not migrate_role.isidentifier():
+                        raise ValueError(f"unsafe migrate role: {migrate_role!r}")
+                    # Leased login roles must SET ROLE to the group so new
+                    # objects are owned by audit_core_migrate, not the lease.
+                    conn.execute(f'SET ROLE "{migrate_role}"')
                 conn.execute(f'CREATE SCHEMA IF NOT EXISTS "{self.schema}"')
                 conn.execute(
                     f'CREATE TABLE IF NOT EXISTS "{self.schema}".schema_migrations ('