feat: 从 gorm 迁移到 ent (#92)

## 主要变更

- 将 ORM 从 GORM 迁移到 Ent
- 使用 SQL 文件迁移替代 GORM AutoMigrate
- 新增迁移运行器支持分布式锁和校验和验证
- 优化 Repository 层查询，新增轻量级存在性检查方法
- 新增完整的单元测试覆盖删除操作

## 迁移优势

- 类型安全与编译期校验
- 关系建模更清晰（Edge/Through）
- 查询一致性更好
- 迁移可控（SQL 文件作为唯一事实来源）
- 可维护性提升

## 新增迁移文件

- 005_schema_parity.sql: 字段对齐
- 006_fix_invalid_subscription_expires_at.sql: 修复过期时间
- 007_add_user_allowed_groups.sql: 用户允许分组表
- 008_seed_default_group.sql: 默认分组种子
- 009_fix_usage_logs_cache_columns.sql: 缓存列修复

backend/ent/predicate/predicate.go

+// 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)