perf(api): align trending ids query with index order

[raymondjacobson]

Summary

• remove a LEFT JOIN from getTrendingIds that did not filter rows
• add a concurrent index matching the existing ORDER BY score DESC, track_id DESC tie-breaker
• keep API ordering behavior unchanged for score ties

Production evidence

• EXPLAIN for the current query planned a nested-loop left join plus incremental sort with startup cost ~309
• removing the inert join and matching index order lets PostgreSQL satisfy the hot lookup from the ordered btree path
• targeted tests caught that track_id DESC is expected for ties, so this PR preserves that contract

Verification

• go test ./api -run 'TestGetTrendingIds|TestGetTrendingUnderground' -count=1 -timeout=2m