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/migrations/embed.go

@@ -1,15 +1,22 @@
 // Package migrations provides the embedded SQL migration files for golang-migrate.
-// Files are named in the golang-migrate convention:
+// Files are named in the golang-migrate convention, under one subdir per dialect:
 //
-//	{version}_{title}.up.sql
-//	{version}_{title}.down.sql
+//	mysql/{version}_{title}.{up,down}.sql
+//	sqlite/{version}_{title}.{up,down}.sql
 //
-// The FS is consumed by internal/store.MigrateUp / MigrateDown.
+// The two FS values are consumed by internal/store.MigrateUp / MigrateDown,
+// selected by the configured DB driver. Version numbers are kept aligned across
+// dialects so schema evolution adds one file to each set.
 package migrations
 
 import "embed"
 
-// FS contains all *.sql files in this directory, embedded at compile time.
+// MySQLFS holds the MySQL-dialect migrations.
 //
-//go:embed *.sql
-var FS embed.FS
+//go:embed mysql/*.sql
+var MySQLFS embed.FS
+
+// SQLiteFS holds the SQLite-dialect migrations.
+//
+//go:embed sqlite/*.sql
+var SQLiteFS embed.FS