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/sqlite/000008_provision.up.sql

@@ -0,0 +1,33 @@
+-- 弹性节点基建。附加式变更。
+ALTER TABLE nodes ADD COLUMN provider_instance_id TEXT NULL;
+ALTER TABLE nodes ADD COLUMN elastic_ip_id        TEXT NULL;
+
+-- node_events.event 的 'ip_rotated' 已在 SQLite 基线（000006）预置，无需扩展。
+
+CREATE TABLE provision_idempotency (
+  idempotency_key TEXT NOT NULL PRIMARY KEY,
+  node_uuid       TEXT NOT NULL,
+  created_at      DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP
+);
+
+CREATE TABLE replacements (
+  uuid        TEXT NOT NULL PRIMARY KEY,
+  old_node_id INTEGER NOT NULL,
+  new_node_id INTEGER NULL,
+  pool        TEXT NOT NULL CHECK (pool IN ('consumable','premium')),
+  status      TEXT NOT NULL DEFAULT 'running' CHECK (status IN ('running','done','failed')),
+  step        TEXT NOT NULL DEFAULT 'open_new',
+  detail      TEXT NULL,                           -- JSON
+  created_at  DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
+  updated_at  DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP
+);
+CREATE INDEX idx_repl_status   ON replacements (status);
+CREATE INDEX idx_repl_old_node ON replacements (old_node_id);
+
+-- 等价 MySQL 的 ON UPDATE CURRENT_TIMESTAMP（recursive_triggers 默认关，不会自递归）。
+CREATE TRIGGER trg_replacements_updated_at
+AFTER UPDATE ON replacements
+FOR EACH ROW
+BEGIN
+  UPDATE replacements SET updated_at = CURRENT_TIMESTAMP WHERE uuid = NEW.uuid;
+END;