Improve cluster allocation strategy.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/migrations/012_cluster_pool_allocation.sql

@@ -12,6 +12,9 @@ ALTER TABLE cluster_pools
   ADD COLUMN IF NOT EXISTS "isDefault" BOOLEAN NOT NULL DEFAULT FALSE,
   ADD COLUMN IF NOT EXISTS priority INTEGER NOT NULL DEFAULT 100;
 
+ALTER TABLE cluster_pools
+  ALTER COLUMN strategy SET DEFAULT 'weighted-resource';
+
 CREATE INDEX IF NOT EXISTS idx_clusters_status_health
   ON clusters(status, "healthStatus");