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/000010_node_blocked_statuses.up.sql

@@ -0,0 +1,13 @@
+-- Extend nodes.status ENUM to include the two intermediate blocking states
+-- added by the detection engine (task 15D).
+--
+-- blocked_suspect  : domestic probes failing + overseas OK for ≥ 2 cycles
+-- blocked_confirmed: suspect held for ≥ 6 cycles → immediately taken off-directory
+--
+-- Note: MySQL ENUM modification rebuilds the table in-place for InnoDB;
+-- on production, run during a low-traffic window.
+ALTER TABLE nodes
+  MODIFY COLUMN status
+    ENUM('provisioning','probing','up','draining','down','destroyed',
+         'blocked_suspect','blocked_confirmed')
+    NOT NULL DEFAULT 'provisioning';