fix(数据库): 修复使用记录缓存列命名不一致

新增迁移补齐 cache_creation_5m_tokens/cache_creation_1h_tokens 列，并从旧列回填数据

backend/migrations/009_fix_usage_logs_cache_columns.sql

+-- Ensure usage_logs cache token columns use the underscored names expected by code.
+-- Backfill from legacy column names if they exist.
+
+ALTER TABLE usage_logs
+    ADD COLUMN IF NOT EXISTS cache_creation_5m_tokens INT NOT NULL DEFAULT 0;
+
+ALTER TABLE usage_logs
+    ADD COLUMN IF NOT EXISTS cache_creation_1h_tokens INT NOT NULL DEFAULT 0;
+
+DO $$
+BEGIN
+    IF EXISTS (
+        SELECT 1
+        FROM information_schema.columns
+        WHERE table_schema = 'public'
+          AND table_name = 'usage_logs'
+          AND column_name = 'cache_creation5m_tokens'
+    ) THEN
+        UPDATE usage_logs
+        SET cache_creation_5m_tokens = cache_creation5m_tokens
+        WHERE cache_creation_5m_tokens = 0
+          AND cache_creation5m_tokens <> 0;
+    END IF;
+
+    IF EXISTS (
+        SELECT 1
+        FROM information_schema.columns
+        WHERE table_schema = 'public'
+          AND table_name = 'usage_logs'
+          AND column_name = 'cache_creation1h_tokens'
+    ) THEN
+        UPDATE usage_logs
+        SET cache_creation_1h_tokens = cache_creation1h_tokens
+        WHERE cache_creation_1h_tokens = 0
+          AND cache_creation1h_tokens <> 0;
+    END IF;
+END $$;