fix: 修复日志重复输出及清理冗余迁移逻辑

- logger: sinkCore 包装 tee core 时绕过了子 core 的 Check 级别过滤，
  导致每条日志同时写入 stdout 和 stderr，表现为启动日志重复显示。
  修复为正确委托 Check 给内部 tee core，sinkCore.Write 仅负责 sink 转发。
- migration 054: 移除冗余的遗留列回填逻辑，migration 009 已完成数据迁移，
  直接删除遗留列即可。

backend/migrations/054_drop_legacy_cache_columns.sql

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