`SET CONSTRAINTS` support

Required for Django migrations. Might be possible to parse the statement and issue a no-op warning. Actual implementation (defer enforcing some constraints until transaction commit) is quite challenging.

No files, tests, or entry points are named. Start by locating parsing and execution for SET CONSTRAINTS, then review how migrations and constraint enforcement handle transactions. Done should provide the required migration behavior, with an explicit decision on whether unsupported cases receive a no-op warning or deferred enforcement.

Written by the indexing model from the issue text.