perf(api): drive usdc purchase tracks from trending scores

[raymondjacobson]

Summary

• removes the materialized usdc_track_ids CTE from /v1/tracks/usdc-purchase
• filters ordered track_trending_scores rows with an EXISTS probe against tracks
• preserves the same eligibility predicates and ORDER BY score DESC, track_id DESC response order

Production evidence

• the database audit found the current plan materializing eligible USDC tracks before sorting joined trending scores, with startup cost around 265k for a small page
• the rewritten shape lets PostgreSQL start from the ordered trending-score path and probe tracks by track_id, which the audit EXPLAIN showed reducing startup work by orders of magnitude for the equivalent ordered scan/probe plan
• this PR is code-only and independent of the separate trending-score index PR

Verification

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