refactor(数据库): 迁移持久层到 Ent 并清理 GORM

将仓储层/基础设施改为 Ent + 原生 SQL 执行路径，并移除 AutoMigrate 与 GORM 依赖。
重构内容包括：
- 仓储层改用 Ent/SQL（含 usage_log/account 等复杂查询），统一错误映射
- 基础设施与 setup 初始化切换为 Ent + SQL migrations
- 集成测试与 fixtures 迁移到 Ent 事务模型
- 清理遗留 GORM 模型/依赖，补充迁移与文档说明
- 增加根目录 Makefile 便于前后端编译

测试：
- go test -tags unit ./...
- go test -tags integration ./...

backend/migrations/001_init.sql

@@ -170,14 +170,3 @@ CREATE INDEX IF NOT EXISTS idx_usage_logs_account_id ON usage_logs(account_id);
 CREATE INDEX IF NOT EXISTS idx_usage_logs_model ON usage_logs(model);
 CREATE INDEX IF NOT EXISTS idx_usage_logs_created_at ON usage_logs(created_at);
 CREATE INDEX IF NOT EXISTS idx_usage_logs_user_created ON usage_logs(user_id, created_at);
-
--- 插入默认管理员用户
--- 密码: admin123 (bcrypt hash)
-INSERT INTO users (email, password_hash, role, balance, concurrency, status)
-VALUES ('admin@sub2api.com', '$2a$10$N9qo8uLOickgx2ZMRZoMye.IjJbDdJeCo0U2bBPJj9lS/5LqD.C.C', 'admin', 0, 10, 'active')
-ON CONFLICT (email) DO NOTHING;
-
--- 插入默认分组
-INSERT INTO groups (name, description, rate_multiplier, is_exclusive, status)
-VALUES ('default', '默认分组', 1.0, false, 'active')
-ON CONFLICT (name) DO NOTHING;