Helps the per-type SUM balance query. M13 (service_items.section_id) and M15
(clinic_doctor_invitations.doctor_id) were false positives — both columns carry
a FK and are therefore auto-indexed by InnoDB; verified against the live schema.
Structural regression: InfraSmokeTest::testWalletUserTypeIndexExists.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>