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/mysql/000007_seed.up.sql

@@ -0,0 +1,19 @@
+-- plans seed（口径来源：design/CLAUDE.md §7，价格不入表）
+-- free: 1 设备，10 分钟/天，每日广告解锁
+-- pro:  5 设备，不限时长，无广告门槛
+-- team: 10 设备，不限时长，无广告门槛
+INSERT INTO plans (code, max_devices, daily_minutes, ad_gate)
+VALUES
+  ('free', 1,  10,   TRUE),
+  ('pro',  5,  NULL, FALSE),
+  ('team', 10, NULL, FALSE)
+ON DUPLICATE KEY UPDATE
+  max_devices   = VALUES(max_devices),
+  daily_minutes = VALUES(daily_minutes),
+  ad_gate       = VALUES(ad_gate);
+
+-- directory_version 单例初始化
+INSERT INTO directory_version (id, version)
+VALUES (1, 1)
+ON DUPLICATE KEY UPDATE
+  version = VALUES(version);