Add track_routes slug allocation index

[raymondjacobson]

Summary

• Add a concurrent index on track_routes (owner_id, title_slug, collision_id DESC).
• Speeds the Core ETL Track Create slug allocation query that does SELECT MAX(collision_id) by owner and title slug before inserting track_routes.

Context

Recent CoreIndexer logs had a Track Create outlier at ~2.46s. The handler calls GenerateSlugAndCollisionID, and the schema only covered track_routes by (owner_id, slug) and track_id; it did not cover the (owner_id, title_slug) lookup used for collision allocation.

Testing

• git diff --check