perf(db): index playlist trending order

[raymondjacobson]

Summary

• adds a concurrent index on playlist_trending_scores(type, version, time_range, score DESC, playlist_id DESC)
• covers /v1/playlists/trending and For You playlist candidate sources without changing API behavior

Production evidence

• the database audit found playlist trending plans doing a parallel scan and sort for the hot ORDER BY score DESC, playlist_id DESC LIMIT ... pattern
• the production-only idx_playlist_trending_scores_filtered puts playlist_id before score, which cannot satisfy the requested ordering
• callers filter by type, version, and time_range, then page by score/tie-breaker; this index follows that shape directly

Verification

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