Convert user admission workflow to use a PostgreSQL stored function to replace the application-layer upsert during invoice confirmation flow.

This is to complete an explicit TODO in payments-service.ts.

Acceptance Criteria

• Create admit_user database function using atomic ON CONFLICT DO UPDATE constraints
• Preserve ASSERT_SERIALIZED() transaction guarantees
• Add Knex migration to provision the function
• Add admitUser interface and DB implementation in UserRepository
• Verify existing repository tests pass

Related

• See TODO in payments-service.ts