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/ent/predicate/predicate.go

@@ -0,0 +1,37 @@
+// Code generated by ent, DO NOT EDIT.
+
+package predicate
+
+import (
+	"entgo.io/ent/dialect/sql"
+)
+
+// Account is the predicate function for account builders.
+type Account func(*sql.Selector)
+
+// AccountGroup is the predicate function for accountgroup builders.
+type AccountGroup func(*sql.Selector)
+
+// ApiKey is the predicate function for apikey builders.
+type ApiKey func(*sql.Selector)
+
+// Group is the predicate function for group builders.
+type Group func(*sql.Selector)
+
+// Proxy is the predicate function for proxy builders.
+type Proxy func(*sql.Selector)
+
+// RedeemCode is the predicate function for redeemcode builders.
+type RedeemCode func(*sql.Selector)
+
+// Setting is the predicate function for setting builders.
+type Setting func(*sql.Selector)
+
+// User is the predicate function for user builders.
+type User func(*sql.Selector)
+
+// UserAllowedGroup is the predicate function for userallowedgroup builders.
+type UserAllowedGroup func(*sql.Selector)
+
+// UserSubscription is the predicate function for usersubscription builders.
+type UserSubscription func(*sql.Selector)