perf(db): drop unused interval play count indexes

[raymondjacobson]

Summary

• drops interval_play_week_count_idx and interval_play_month_count_idx concurrently when present
• leaves interval_play_track_id_idx, which covers API joins by track

Production evidence

• the database audit found both count-only aggregate interval play indexes with idx_scan = 0
• API reads join aggregate play data by track_id; the checked-in schema keeps interval_play_track_id_idx for that path
• dropping unused indexes reduces write amplification and autovacuum/index-maintenance work on aggregate play updates

Verification

• migration review only; SQL is DROP INDEX CONCURRENTLY IF EXISTS
• no application code changed