MARS: FK covering indexes for Disk IO budget alert

53 FK covering indexes (mig 167) for get_advisors unindexed_foreign_keys findings, motivated by Supabase Disk IO budget alert. Applied prod, self-verified count=53, audit-general-cc PASS:45f39b8, pushed 333418f v2.79.85 live-confirmed by coder-mars-cc. 27 unused-index candidates surveyed, not dropped (total 3.5MB, low value/risk).