PLUTO-751: FK covering indexes (76, advisor unindexed_foreign_keys)

Migration 122 written: db/migrations/applied/122-pluto-751-fk-covering-indexes.sql. Mirrors MARS-754/mig167. All target tables tiny (practicas 1411 rows largest per db-pluto-cc), plain CREATE INDEX IF NOT EXISTS in one txn, lock_timeout 5s, post-check asserts count=76. Not yet applied to prod - needs db-pluto-cc to run via apply_migration and pnpm db:export after. unused_index=19 from same advisor call is out of scope, flagged not asked.