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/000002_plans_subscriptions.up.sql

@@ -0,0 +1,20 @@
+-- 套餐与订阅
+CREATE TABLE plans (
+  id            INTEGER PRIMARY KEY AUTOINCREMENT,
+  code          TEXT NOT NULL UNIQUE CHECK (code IN ('free','pro','team')),
+  max_devices   INTEGER NOT NULL,
+  daily_minutes INTEGER NULL,
+  ad_gate       INTEGER NOT NULL DEFAULT 0
+);
+
+CREATE TABLE subscriptions (
+  id          INTEGER PRIMARY KEY AUTOINCREMENT,
+  user_id     INTEGER NOT NULL,
+  plan_id     INTEGER NOT NULL,
+  expires_at  DATETIME NOT NULL,
+  source      TEXT NOT NULL CHECK (source IN ('trial','code')),
+  created_at  DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
+  FOREIGN KEY (user_id) REFERENCES users(id),
+  FOREIGN KEY (plan_id) REFERENCES plans(id)
+);
+CREATE INDEX idx_subs_user_exp ON subscriptions (user_id, expires_at);