fix: 修改创建索引逻辑

Author: quantal

backend/internal/bootstrap/migrations/20260528_usage_logs_metadata_and_created_at_index.sql

@@ -25,5 +25,4 @@ ALTER TABLE public.usage_logs DROP COLUMN IF EXISTS usage_attributes;
 ALTER TABLE public.usage_logs DROP COLUMN IF EXISTS usage_metrics;
 ALTER TABLE public.usage_logs DROP COLUMN IF EXISTS usage_cost_details;
 
-DROP INDEX CONCURRENTLY IF EXISTS public.usage_log_created_at;
-CREATE INDEX CONCURRENTLY usage_log_created_at ON public.usage_logs (created_at);
+CREATE INDEX CONCURRENTLY IF NOT EXISTS usage_log_created_at ON public.usage_logs (created_at);