MARS-117 Mars: track index/DDL changes in-repo — schema.md omits indexes (0 index refs) and DDL is applied via Supabase MCP (recorded only in Supabase migrations table), so index changes like ix_appEvents_category_action_createdAt have no git-side record. Add index export to export-db.py OR adopt a db/migrations file convention so DDL is reproducible/reviewable from the repo.