perf: reduce trending job score writes

[raymondjacobson]

Summary

• Persist only positive track trending scores while keeping the existing score expressions intact, reducing the zero-score tail rewritten by TrendingJob.
• Drop superseded ascending-tiebreak track trending indexes now covered by desc-tiebreak indexes.
• Align challenge/tastemaker trending reads with the desc-tiebreak index order and add tests for positive-row persistence plus zero-score pruning.

Production evidence

• track_trending_scores is about 12.2M rows / 17GB total in prod, with about 16GB of indexes.
• Read-only prod counts showed only about 2.31M positive track score rows; week/month ranges are overwhelmingly zero-score rows.
• score is NOT NULL in prod, so removing NULLS LAST from challenge reads should preserve ordering while letting Postgres use the existing desc-tiebreak indexes.

Compatibility note

• This intentionally stops returning arbitrary zero-score filler rows from track_trending_scores. Shallow trending reads should be unaffected, but very deep pagination or sparse genre queries may return fewer rows after all positive scores are exhausted.

Test plan

• go test ./jobs -run TestTrendingJob -count=1
• go test ./jobs/challenges -run "TestTrending|TestTastemaker" -count=1
• go test ./api -run "Trending|RecommendedTracks|GenreTop" -count=1
• git diff --check