fix(gateway): 避免SSE delta将缓存创建明细重置为0

- 仅在 delta 中 5m/1h 值大于0时覆盖 usage 明细
- 新增回归测试覆盖 delta 默认 0 不应覆盖 message_start 非零值
- 迁移 054 在删除 legacy 字段前追加一次回填，避免升级实例丢失历史写入

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/migrations/054_drop_legacy_cache_columns.sql

@@ -8,7 +8,37 @@
 --   cache_creation_1h_tokens  (defined in 001_init.sql)
 --
 -- Migration 009 already copied data from legacy → canonical columns.
--- This migration drops the legacy columns to avoid confusion.
+-- But upgraded instances may still have post-009 writes in legacy columns.
+-- Backfill once more before dropping to prevent data loss.
+
+DO $$
+BEGIN
+    IF EXISTS (
+        SELECT 1
+        FROM information_schema.columns
+        WHERE table_schema = 'public'
+          AND table_name = 'usage_logs'
+          AND column_name = 'cache_creation5m_tokens'
+    ) THEN
+        UPDATE usage_logs
+        SET cache_creation_5m_tokens = cache_creation5m_tokens
+        WHERE cache_creation_5m_tokens = 0
+          AND cache_creation5m_tokens <> 0;
+    END IF;
+
+    IF EXISTS (
+        SELECT 1
+        FROM information_schema.columns
+        WHERE table_schema = 'public'
+          AND table_name = 'usage_logs'
+          AND column_name = 'cache_creation1h_tokens'
+    ) THEN
+        UPDATE usage_logs
+        SET cache_creation_1h_tokens = cache_creation1h_tokens
+        WHERE cache_creation_1h_tokens = 0
+          AND cache_creation1h_tokens <> 0;
+    END IF;
+END $$;
 
 ALTER TABLE usage_logs DROP COLUMN IF EXISTS cache_creation5m_tokens;
 ALTER TABLE usage_logs DROP COLUMN IF EXISTS cache_creation1h_tokens;