V69: Make legacy integer FK columns nullable on role tables.

After the UUID cleanup, role schemas only write user_uuid / role_uuid — they never set the legacy integer FK columns. Two tables still have these columns as NOT NULL, causing inserts to fail with not_null_violation.

This was missed in V67 (the broader NOT NULL cleanup migration).

Columns Fixed

• phoenix_kit_user_role_assignments.user_id (NOT NULL → nullable)
• phoenix_kit_user_role_assignments.role_id (NOT NULL → nullable)
• phoenix_kit_role_permissions.role_id (NOT NULL → nullable)

All operations are idempotent (guarded by table/column existence + NOT NULL checks).