Add track_routes slug allocation index

ddl/migrations/0225_track_routes_slug_allocation_idx.sql

@@ -0,0 +1,12 @@
+-- Supports ETL track route allocation:
+--
+--   SELECT MAX(collision_id)
+--   FROM track_routes
+--   WHERE owner_id = ?
+--     AND title_slug = ?
+--
+-- The primary key covers (owner_id, slug), but not title_slug. Track creates
+-- run this lookup before inserting each track_routes row, so give Postgres a
+-- narrow index for the slug-allocation path.
+CREATE INDEX CONCURRENTLY IF NOT EXISTS track_routes_owner_title_slug_collision_idx
+    ON public.track_routes USING btree (owner_id, title_slug, collision_id DESC);