feat(server/db): 数据层多库支持(1/4)— 连接分派 + 双方言迁移管线

- config 增 DB_DRIVER(mysql 默认 | sqlite);DSN 对 sqlite 为文件路径
- db.OpenDriver 按驱动分派:sqlite 用 modernc(纯 Go 免 CGO)+ WAL/
  busy_timeout/foreign_keys/_txlock=immediate;mysql 路径不变
- store.Open 分派;mysql 保留 UTC/collation 断言,sqlite 跳过
- 迁移拆 migrations/{mysql,sqlite}/ 双套,embed 双 FS,migrate 按驱动选源
  与 golang-migrate 驱动;修复 m.Close() 误关调用方 *sql.DB 的坑
- cmd/migrate 串入 DB_DRIVER;集成测试 MigrateUp 签名更新
- 新增 SQLite 时间往返 smoke 测试与端到端迁移测试(免 docker)

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

server/cmd/migrate/main.go

@@ -27,12 +27,13 @@ func main() {
 	}
 	cmd := os.Args[1]
 
+	driver := os.Getenv("DB_DRIVER")
 	dsn := os.Getenv("DB_DSN")
 	if dsn == "" {
 		log.Fatal("migrate: DB_DSN is required")
 	}
 
-	db, err := store.Open(&config.Config{DSN: dsn})
+	db, err := store.Open(&config.Config{Driver: driver, DSN: dsn})
 	if err != nil {
 		log.Fatalf("migrate: db: %v", err)
 	}
@@ -40,19 +41,19 @@ func main() {
 
 	switch cmd {
 	case "up":
-		if err := store.MigrateUp(db); err != nil {
+		if err := store.MigrateUp(db, driver); err != nil {
 			log.Fatalf("migrate up: %v", err)
 		}
 		log.Println("migrate: up — done")
 
 	case "down":
-		if err := store.MigrateDown(db); err != nil {
+		if err := store.MigrateDown(db, driver); err != nil {
 			log.Fatalf("migrate down: %v", err)
 		}
 		log.Println("migrate: down — done")
 
 	case "version":
-		version, dirty, err := store.MigrateVersion(db)
+		version, dirty, err := store.MigrateVersion(db, driver)
 		if err != nil {
 			log.Fatalf("migrate version: %v", err)
 		}
@@ -69,5 +70,6 @@ func usage() {
 	fmt.Fprintln(os.Stderr, "Usage: migrate <up|down|version>")
 	fmt.Fprintln(os.Stderr, "")
 	fmt.Fprintln(os.Stderr, "Environment variables:")
-	fmt.Fprintln(os.Stderr, "  DB_DSN   full MySQL DSN (required)")
+	fmt.Fprintln(os.Stderr, "  DB_DRIVER  mysql (default) | sqlite")
+	fmt.Fprintln(os.Stderr, "  DB_DSN     mysql: full DSN · sqlite: file path or :memory: (required)")
 }