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/000004_usage_audit.up.sql

@@ -0,0 +1,21 @@
+-- 用量（仅字节/分钟，无目的地）
+CREATE TABLE usage_daily (
+  user_id        INTEGER NOT NULL,
+  date           DATE NOT NULL,
+  bytes_up       INTEGER NOT NULL DEFAULT 0,
+  bytes_down     INTEGER NOT NULL DEFAULT 0,
+  minutes_used   INTEGER NOT NULL DEFAULT 0,
+  ad_unlocked_at DATETIME NULL,
+  PRIMARY KEY (user_id, date)
+);
+
+-- 审计
+CREATE TABLE audit_log (
+  id      INTEGER PRIMARY KEY AUTOINCREMENT,
+  actor   TEXT NOT NULL,
+  action  TEXT NOT NULL,
+  target  TEXT NOT NULL,
+  meta    TEXT NULL,                               -- JSON
+  at      DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP
+);
+CREATE INDEX idx_audit_at ON audit_log (at);