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/accountgroup_delete.go

+// Code generated by ent, DO NOT EDIT.
+
+package ent
+
+import (
+	"context"
+
+	"entgo.io/ent/dialect/sql"
+	"entgo.io/ent/dialect/sql/sqlgraph"
+	"github.com/Wei-Shaw/sub2api/ent/accountgroup"
+	"github.com/Wei-Shaw/sub2api/ent/predicate"
+)
+
+// AccountGroupDelete is the builder for deleting a AccountGroup entity.
+type AccountGroupDelete struct {
+	config
+	hooks    []Hook
+	mutation *AccountGroupMutation
+}
+
+// Where appends a list predicates to the AccountGroupDelete builder.
+func (_d *AccountGroupDelete) Where(ps ...predicate.AccountGroup) *AccountGroupDelete {
+	_d.mutation.Where(ps...)
+	return _d
+}
+
+// Exec executes the deletion query and returns how many vertices were deleted.
+func (_d *AccountGroupDelete) Exec(ctx context.Context) (int, error) {
+	return withHooks(ctx, _d.sqlExec, _d.mutation, _d.hooks)
+}
+
+// ExecX is like Exec, but panics if an error occurs.
+func (_d *AccountGroupDelete) ExecX(ctx context.Context) int {
+	n, err := _d.Exec(ctx)
+	if err != nil {
+		panic(err)
+	}
+	return n
+}
+
+func (_d *AccountGroupDelete) sqlExec(ctx context.Context) (int, error) {
+	_spec := sqlgraph.NewDeleteSpec(accountgroup.Table, nil)
+	if ps := _d.mutation.predicates; len(ps) > 0 {
+		_spec.Predicate = func(selector *sql.Selector) {
+			for i := range ps {
+				ps[i](selector)
+			}
+		}
+	}
+	affected, err := sqlgraph.DeleteNodes(ctx, _d.driver, _spec)
+	if err != nil && sqlgraph.IsConstraintError(err) {
+		err = &ConstraintError{msg: err.Error(), wrap: err}
+	}
+	_d.mutation.done = true
+	return affected, err
+}
+
+// AccountGroupDeleteOne is the builder for deleting a single AccountGroup entity.
+type AccountGroupDeleteOne struct {
+	_d *AccountGroupDelete
+}
+
+// Where appends a list predicates to the AccountGroupDelete builder.
+func (_d *AccountGroupDeleteOne) Where(ps ...predicate.AccountGroup) *AccountGroupDeleteOne {
+	_d._d.mutation.Where(ps...)
+	return _d
+}
+
+// Exec executes the deletion query.
+func (_d *AccountGroupDeleteOne) Exec(ctx context.Context) error {
+	n, err := _d._d.Exec(ctx)
+	switch {
+	case err != nil:
+		return err
+	case n == 0:
+		return &NotFoundError{accountgroup.Label}
+	default:
+		return nil
+	}
+}
+
+// ExecX is like Exec, but panics if an error occurs.
+func (_d *AccountGroupDeleteOne) ExecX(ctx context.Context) {
+	if err := _d.Exec(ctx); err != nil {
+		panic(err)
+	}
+}

backend/ent/driver_access.go

+package ent
+
+import "entgo.io/ent/dialect"
+
+// Driver 暴露底层 driver，供需要 raw SQL 的集成层使用。
+func (c *Client) Driver() dialect.Driver {
+	return c.driver
+}

backend/ent/generate.go

+package ent
+
+// 启用 sql/execquery 以生成 ExecContext/QueryContext 的透传接口，便于事务内执行原生 SQL。
+//go:generate go run -mod=mod entgo.io/ent/cmd/ent generate --feature sql/upsert,intercept,sql/execquery --idtype int64 ./schema