Merge pull request #875 from mt21625457/fix/openai-fast-billing-clean

fix(billing): 修复 OpenAI fast 档位计费并补齐展示

backend/migrations/070_add_usage_log_service_tier.sql

@@ -0,0 +1,5 @@
+ALTER TABLE usage_logs
+    ADD COLUMN IF NOT EXISTS service_tier VARCHAR(16);
+
+CREATE INDEX IF NOT EXISTS idx_usage_logs_service_tier_created_at
+    ON usage_logs (service_tier, created_at);