perf(api): simplify underground trending joins

[raymondjacobson]

Summary

• removes a non-filtering tracks join from the underground exclusion CTE
• makes the main tracks and aggregate_user joins explicit inner joins, matching the existing follower/following predicates

Production evidence

• /v1/tracks/trending/underground was one of the hot trending endpoints in the database audit
• the old CTE joined tracks with liveness predicates in the ON clause but selected only track_trending_scores.track_id, so deleted/unlisted/unavailable tracks were still counted in the top-20 exclusion set while adding work
• the main query already rejected NULL aggregate_user rows via aggregate_user.follower_count < 1500 and following_count < 1500, so explicit joins give the planner a simpler join graph with the same endpoint behavior

Verification

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