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/devices/store.go

@@ -5,6 +5,8 @@ import (
 	"database/sql"
 	"fmt"
 	"time"
+
+	dbx "github.com/wangjia/pangolin/server/internal/db"
 )
 
 // DeviceRow mirrors a `devices` table row.
@@ -33,11 +35,12 @@ type effSub struct {
 // Store wraps a *sql.DB and exposes the database operations the devices module
 // needs. Methods that take a *sql.Tx run within that transaction.
 type Store struct {
-	db *sql.DB
+	db      *sql.DB
+	dialect dbx.Dialect
 }
 
-// NewStore creates a Store backed by the given MySQL connection pool.
-func NewStore(db *sql.DB) *Store { return &Store{db: db} }
+// NewStore creates a Store backed by the given connection pool (MySQL or SQLite).
+func NewStore(db *sql.DB) *Store { return &Store{db: db, dialect: dbx.DialectForDB(db)} }
 
 // BeginTx starts a transaction at Read Committed isolation. Per-user
 // serialization for device mutations is achieved by locking the users row
@@ -77,7 +80,7 @@ func (s *Store) ListByUser(ctx context.Context, userID int64) ([]DeviceRow, erro
 func (s *Store) findDeviceByUUIDTx(ctx context.Context, tx *sql.Tx, uuid string) (*DeviceRow, error) {
 	row := tx.QueryRowContext(ctx,
 		`SELECT id, uuid, user_id, name, platform, last_seen, created_at
-		 FROM devices WHERE uuid=? FOR UPDATE`, uuid)
+		 FROM devices WHERE uuid=? `+s.dialect.LockForUpdate(), uuid)
 	var d DeviceRow
 	if err := row.Scan(&d.ID, &d.UUID, &d.UserID, &d.Name, &d.Platform, &d.LastSeen, &d.CreatedAt); err == sql.ErrNoRows {
 		return nil, nil
@@ -90,7 +93,7 @@ func (s *Store) findDeviceByUUIDTx(ctx context.Context, tx *sql.Tx, uuid string)
 // lockUser locks the users row to serialize per-user device mutations and
 // returns the user's status. Returns (false, "", nil) when the user is absent.
 func (s *Store) lockUser(ctx context.Context, tx *sql.Tx, userID int64) (exists bool, status string, err error) {
-	row := tx.QueryRowContext(ctx, `SELECT status FROM users WHERE id=? FOR UPDATE`, userID)
+	row := tx.QueryRowContext(ctx, `SELECT status FROM users WHERE id=? `+s.dialect.LockForUpdate(), userID)
 	if e := row.Scan(&status); e == sql.ErrNoRows {
 		return false, "", nil
 	} else if e != nil {
@@ -110,10 +113,11 @@ func (s *Store) countDevicesTx(ctx context.Context, tx *sql.Tx, userID int64) (i
 
 // insertDeviceTx inserts a new device row inside tx and returns it.
 func (s *Store) insertDeviceTx(ctx context.Context, tx *sql.Tx, uuid string, userID int64, name, platform string) (*DeviceRow, error) {
+	now := time.Now().UTC()
 	res, err := tx.ExecContext(ctx,
 		`INSERT INTO devices (uuid, user_id, name, platform, last_seen, created_at)
-		 VALUES (?, ?, ?, ?, UTC_TIMESTAMP(6), UTC_TIMESTAMP(6))`,
-		uuid, userID, name, platform)
+		 VALUES (?, ?, ?, ?, ?, ?)`,
+		uuid, userID, name, platform, now, now)
 	if err != nil {
 		return nil, fmt.Errorf("store.insertDeviceTx: %w", err)
 	}
@@ -124,7 +128,7 @@ func (s *Store) insertDeviceTx(ctx context.Context, tx *sql.Tx, uuid string, use
 // touchLastSeenTx updates a device's last_seen to now inside tx.
 func (s *Store) touchLastSeenTx(ctx context.Context, tx *sql.Tx, deviceID int64) error {
 	_, err := tx.ExecContext(ctx,
-		`UPDATE devices SET last_seen=UTC_TIMESTAMP(6) WHERE id=?`, deviceID)
+		`UPDATE devices SET last_seen=? WHERE id=?`, time.Now().UTC(), deviceID)
 	if err != nil {
 		return fmt.Errorf("store.touchLastSeenTx: %w", err)
 	}
@@ -212,8 +216,8 @@ func (s *Store) writeAuditLogTx(ctx context.Context, tx *sql.Tx, actor, action,
 	}
 	_, err := tx.ExecContext(ctx,
 		`INSERT INTO audit_log (actor, action, target, meta, at)
-		 VALUES (?, ?, ?, ?, UTC_TIMESTAMP(6))`,
-		actor, action, target, metaJSON)
+		 VALUES (?, ?, ?, ?, ?)`,
+		actor, action, target, metaJSON, time.Now().UTC())
 	if err != nil {
 		return fmt.Errorf("store.writeAuditLogTx: %w", err)
 	}