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/provision/mysqlstore.go

@@ -6,15 +6,22 @@ import (
 	"encoding/json"
 	"fmt"
 	"strings"
+	"time"
+
+	dbx "github.com/wangjia/pangolin/server/internal/db"
 )
 
 // MySQLStore is the production Store backed by the shared *sql.DB pool.
+// (Despite the name it is dialect-aware and also works against SQLite.)
 type MySQLStore struct {
-	db *sql.DB
+	db      *sql.DB
+	dialect dbx.Dialect
 }
 
-// NewMySQLStore wraps a MySQL connection pool.
-func NewMySQLStore(db *sql.DB) *MySQLStore { return &MySQLStore{db: db} }
+// NewMySQLStore wraps a database connection pool (MySQL or SQLite).
+func NewMySQLStore(db *sql.DB) *MySQLStore {
+	return &MySQLStore{db: db, dialect: dbx.DialectForDB(db)}
+}
 
 var _ Store = (*MySQLStore)(nil)
 
@@ -54,10 +61,10 @@ func (s *MySQLStore) InsertNode(ctx context.Context, n *Node) (int64, error) {
 		`INSERT INTO nodes
 		  (uuid, region, name_zh, name_en, role, tier, endpoint, hy2_port,
 		   reality_pbk, reality_sni, provider_id, tags, status, weight, created_at)
-		 VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, UTC_TIMESTAMP(6))`,
+		 VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)`,
 		n.UUID, n.Region, n.NameZH, n.NameEn, string(n.Role), string(n.Tier),
 		endpoint, nullInt(n.HY2Port), n.RealityPBK, n.RealitySNI, n.ProviderID,
-		marshalJSONList(n.Tags), string(status), weight)
+		marshalJSONList(n.Tags), string(status), weight, time.Now().UTC())
 	if err != nil {
 		return 0, fmt.Errorf("store.InsertNode: %w", err)
 	}
@@ -236,8 +243,8 @@ func (s *MySQLStore) WriteNodeEvent(ctx context.Context, nodeID int64, event Eve
 		detailJSON = "null"
 	}
 	_, err := s.db.ExecContext(ctx,
-		`INSERT INTO node_events (node_id, event, detail, at) VALUES (?, ?, ?, UTC_TIMESTAMP(6))`,
-		nodeID, string(event), detailJSON)
+		`INSERT INTO node_events (node_id, event, detail, at) VALUES (?, ?, ?, ?)`,
+		nodeID, string(event), detailJSON, time.Now().UTC())
 	if err != nil {
 		return fmt.Errorf("store.WriteNodeEvent: %w", err)
 	}
@@ -249,8 +256,8 @@ func (s *MySQLStore) WriteAuditLog(ctx context.Context, actor, action, target, m
 		metaJSON = "null"
 	}
 	_, err := s.db.ExecContext(ctx,
-		`INSERT INTO audit_log (actor, action, target, meta, at) VALUES (?, ?, ?, ?, UTC_TIMESTAMP(6))`,
-		actor, action, target, metaJSON)
+		`INSERT INTO audit_log (actor, action, target, meta, at) VALUES (?, ?, ?, ?, ?)`,
+		actor, action, target, metaJSON, time.Now().UTC())
 	if err != nil {
 		return fmt.Errorf("store.WriteAuditLog: %w", err)
 	}
@@ -288,9 +295,9 @@ func (s *MySQLStore) LookupIdempotency(ctx context.Context, key string) (string,
 func (s *MySQLStore) SaveIdempotency(ctx context.Context, key, nodeUUID string) error {
 	_, err := s.db.ExecContext(ctx,
 		`INSERT INTO provision_idempotency (idempotency_key, node_uuid, created_at)
-		 VALUES (?, ?, UTC_TIMESTAMP(6))
-		 ON DUPLICATE KEY UPDATE idempotency_key = idempotency_key`,
-		key, nodeUUID)
+		 VALUES (?, ?, ?) `+
+			s.dialect.Upsert([]string{"idempotency_key"}),
+		key, nodeUUID, time.Now().UTC())
 	if err != nil {
 		return fmt.Errorf("store.SaveIdempotency: %w", err)
 	}