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/go.mod

@@ -22,6 +22,8 @@ require (
 	google.golang.org/grpc v1.81.1
 	google.golang.org/grpc/cmd/protoc-gen-go-grpc v1.1.0
 	google.golang.org/protobuf v1.36.11
+	gopkg.in/yaml.v3 v3.0.1
+	modernc.org/sqlite v1.18.1
 )
 
 require (
@@ -281,7 +283,6 @@ require (
 	google.golang.org/genproto/googleapis/api v0.0.0-20260523011958-0a33c5d7ca68 // indirect
 	google.golang.org/genproto/googleapis/rpc v0.0.0-20260523011958-0a33c5d7ca68 // indirect
 	gopkg.in/inf.v0 v0.9.1 // indirect
-	gopkg.in/yaml.v3 v3.0.1 // indirect
 	lukechampine.com/uint128 v1.2.0 // indirect
 	modernc.org/b v1.0.0 // indirect
 	modernc.org/cc/v3 v3.36.3 // indirect
@@ -298,7 +299,6 @@ require (
 	modernc.org/opt v0.1.3 // indirect
 	modernc.org/ql v1.0.0 // indirect
 	modernc.org/sortutil v1.1.0 // indirect
-	modernc.org/sqlite v1.18.1 // indirect
 	modernc.org/strutil v1.1.3 // indirect
 	modernc.org/token v1.0.0 // indirect
 	modernc.org/zappy v1.0.0 // indirect