perf(db): index for you trending tie order

[raymondjacobson]

Summary

• adds a concurrent partial index for the For You weekly track candidate slice
• matches the endpoint order exactly: score DESC, track_id DESC
• leaves the existing partial index in place so this can roll back independently

Production evidence

• the database audit found the For You weekly trending/underground candidate plan using the existing partial index but still doing an incremental sort for tie ordering
• api/v1_users_feed_for_you.go orders both cand_trending_track and cand_underground_track by tts.score DESC, tts.track_id DESC
• the existing idx_track_trending_scores_for_you index is (score DESC, track_id) and cannot satisfy that tie direction directly

Verification

• migration review only; SQL is CREATE INDEX CONCURRENTLY IF NOT EXISTS plus an index comment
• no application code changed