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/internal/store/mysql.go

@@ -5,60 +5,55 @@ import (
 	"fmt"
 	"time"
 
-	// Register the mysql driver with database/sql.
+	// mysql.ParseDSN is used to structurally rewrite the DSN for UTC.
 	"github.com/go-sql-driver/mysql"
-	_ "github.com/go-sql-driver/mysql"
 
 	"github.com/wangjia/pangolin/server/internal/config"
+	"github.com/wangjia/pangolin/server/internal/db"
 )
 
-// Open is the single canonical MySQL entry point for the Pangolin server.
+// Open is the canonical database entry point for the Pangolin server. It
+// dispatches on cfg.Driver:
 //
-// It:
-//  1. Parses the DSN from cfg.DSN.
-//  2. Structurally overrides UTC parameters (ParseTime, Loc, Collation,
-//     time_zone session variable) — no string manipulation.
-//  3. Configures the connection pool.
-//  4. Pings the server.
-//  5. Asserts SELECT @@session.time_zone = "+00:00" (startup fatal if the
-//     server ignores our DSN override — e.g. SQL mode forces a different TZ).
+//   - sqlite: opens the file/in-memory DB (UTC is native; no session assertion).
+//   - mysql (default): structurally overrides the DSN for UTC (ParseTime, Loc,
+//     Collation, time_zone), opens the pool, and asserts
+//     SELECT @@session.time_zone = "+00:00" (startup-fatal if the server ignores
+//     our UTC override — e.g. SQL mode forces a different TZ).
+//
+// The actual connection/pool/ping lives in internal/db; this layer adds only the
+// MySQL-specific UTC rigor.
 func Open(cfg *config.Config) (*sql.DB, error) {
+	if db.Normalize(cfg.Driver) == "sqlite" {
+		return db.OpenDriver("sqlite", cfg.DSN)
+	}
+
 	dsn, err := buildDSN(cfg)
 	if err != nil {
 		return nil, fmt.Errorf("store.Open: %w", err)
 	}
 
-	db, err := sql.Open("mysql", dsn)
+	database, err := db.OpenDriver("mysql", dsn)
 	if err != nil {
 		return nil, fmt.Errorf("store.Open: %w", err)
 	}
 
-	// Connection pool.
-	db.SetMaxOpenConns(30)
-	db.SetMaxIdleConns(10)
-	db.SetConnMaxLifetime(5 * time.Minute)
-
-	if err := db.Ping(); err != nil {
-		_ = db.Close()
-		return nil, fmt.Errorf("store.Open: ping: %w", err)
-	}
-
 	// Hard assertion: the session time_zone must be "+00:00".
 	// This catches MySQL servers that ignore client-supplied time_zone params.
 	var tz string
-	if err := db.QueryRow("SELECT @@session.time_zone").Scan(&tz); err != nil {
-		_ = db.Close()
+	if err := database.QueryRow("SELECT @@session.time_zone").Scan(&tz); err != nil {
+		_ = database.Close()
 		return nil, fmt.Errorf("store.Open: query @@session.time_zone: %w", err)
 	}
 	if tz != "+00:00" {
-		_ = db.Close()
+		_ = database.Close()
 		return nil, fmt.Errorf(
 			"store.Open: session time_zone=%q, want +00:00; UTC DSN override failed",
 			tz,
 		)
 	}
 
-	return db, nil
+	return database, nil
 }
 
 // buildDSN parses cfg.DSN and structurally overrides UTC and utf8mb4