* perf(db): optimize D1 indexes to eliminate full table scans
- Add composite indexes to ec_* tables for common query patterns
- Replace single-column indexes with (deleted_at, updated_at, id) composite
- Add (deleted_at, status) index for count queries
- Add (deleted_at, created_at, id) index for chronological ordering
- Optimize comment counting with partial indexes per status
- Rewrite countByStatus() to use parallel WHERE queries instead of GROUP BY
Fixes#131
* chore: add changeset for D1 index optimization
* style: wrap changeset description for formatting compliance
