perf(db): drop unused balance history mint index

[raymondjacobson]

Summary

• drops user_balance_history_user_mint_timestamp_idx concurrently
• keeps user_balance_history_user_timestamp_idx, which covers the API endpoint predicate

Production evidence

• the database audit found user_balance_history_user_mint_timestamp_idx at roughly 78 GB with idx_scan = 0
• api/v1_users_balance_history.go filters by user_id and timestamp, not mint
• dropping the unused wider index removes write amplification, vacuum overhead, and cache pressure from a large table

Verification

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