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_integration_test.go

@@ -53,11 +53,11 @@ func TestIntegration_TimeZoneAssertionAndMigrateUp(t *testing.T) {
 	defer db.Close()
 
 	// ── 2. First MigrateUp ───────────────────────────────────────────────────
-	if err := store.MigrateUp(db); err != nil {
+	if err := store.MigrateUp(db, "mysql"); err != nil {
 		t.Fatalf("MigrateUp (first): %v", err)
 	}
 
-	v, dirty, err := store.MigrateVersion(db)
+	v, dirty, err := store.MigrateVersion(db, "mysql")
 	if err != nil {
 		t.Fatalf("MigrateVersion after up: %v", err)
 	}
@@ -70,16 +70,16 @@ func TestIntegration_TimeZoneAssertionAndMigrateUp(t *testing.T) {
 	t.Logf("after MigrateUp: version=%d dirty=%v", v, dirty)
 
 	// ── 3. Idempotent second MigrateUp ───────────────────────────────────────
-	if err := store.MigrateUp(db); err != nil {
+	if err := store.MigrateUp(db, "mysql"); err != nil {
 		t.Fatalf("MigrateUp (idempotent): %v", err)
 	}
 
 	// ── 4. MigrateDown ───────────────────────────────────────────────────────
-	if err := store.MigrateDown(db); err != nil {
+	if err := store.MigrateDown(db, "mysql"); err != nil {
 		t.Fatalf("MigrateDown: %v", err)
 	}
 
-	v2, _, err := store.MigrateVersion(db)
+	v2, _, err := store.MigrateVersion(db, "mysql")
 	if err != nil {
 		t.Fatalf("MigrateVersion after down: %v", err)
 	}