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/config/config.go

@@ -10,8 +10,13 @@ import (
 
 // Config holds all application-level configuration.
 type Config struct {
-	// DSN is the MySQL connection string.
-	// Format: user:password@tcp(host:port)/dbname?parseTime=true&loc=UTC&time_zone=%%27UTC%%27
+	// Driver selects the database engine: "mysql" (default) or "sqlite".
+	// Read from DB_DRIVER. Switching engines is a single env var change.
+	Driver string
+
+	// DSN is the database connection string.
+	//   mysql:  user:password@tcp(host:port)/dbname?parseTime=true&loc=UTC&time_zone=%%27UTC%%27
+	//   sqlite: a file path (e.g. /var/lib/pangolin/pangolin.db) or :memory:
 	DSN string
 
 	// RedisAddr is the Redis server address (host:port).
@@ -72,6 +77,7 @@ type Config struct {
 // Returns an error if required variables are missing.
 func FromEnv() (*Config, error) {
 	c := &Config{
+		Driver:                    os.Getenv("DB_DRIVER"),
 		DSN:                       os.Getenv("DB_DSN"),
 		RedisAddr:                 getEnvDefault("REDIS_ADDR", "127.0.0.1:6379"),
 		RedisPassword:             os.Getenv("REDIS_PASSWORD"),