feat(server/db): 多库(3/4)— Dialect 抽象(upsert/锁)+ 合并 directory_version

- db.Dialect:LockForUpdate(mysql "FOR UPDATE" / sqlite "")、Upsert(中性
  EXCLUDED.col → mysql VALUES()/ sqlite excluded.);DialectForDB 从连接驱动推导
- 9 处 ON DUPLICATE KEY、11 处 FOR UPDATE 全走 dialect;sqlite 靠 _txlock=
  immediate 取得 BEGIN IMMEDIATE 悲观锁等价语义
- directory_version 三处重复合并为 store.BumpDirectoryVersion(dialect 感知)
- 这些文件同时含(2/4)的 UTC→Go 改动(与 upsert/锁同语句交错,无法拆分)
- 顺带:usage 的 FIELD()、codes 的 DATE_ADD/GREATEST 续期、nodes 的
  UNIX_TIMESTAMP、NULLIF 等 MySQL 专属构造一并退回 Go/可移植写法

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

server/internal/store/directory.go

@@ -0,0 +1,28 @@
+package store
+
+import (
+	"context"
+	"database/sql"
+	"fmt"
+
+	"github.com/wangjia/pangolin/server/internal/db"
+)
+
+// Execer is satisfied by both *sql.DB and *sql.Tx.
+type Execer interface {
+	ExecContext(ctx context.Context, query string, args ...any) (sql.Result, error)
+}
+
+// BumpDirectoryVersion atomically increments the directory_version singleton
+// (the row is seeded by migration 7). This is the single canonical
+// implementation; nodes.Lifecycle and the scheduler both call it instead of
+// each carrying their own copy of the upsert.
+func BumpDirectoryVersion(ctx context.Context, q Execer, d db.Dialect) error {
+	_, err := q.ExecContext(ctx,
+		`INSERT INTO directory_version (id, version) VALUES (1, 1) `+
+			d.Upsert([]string{"id"}, "version = version + 1"))
+	if err != nil {
+		return fmt.Errorf("store.BumpDirectoryVersion: %w", err)
+	}
+	return nil
+}