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/account_delete.go

@@ -0,0 +1,88 @@
+// Code generated by ent, DO NOT EDIT.
+
+package ent
+
+import (
+	"context"
+
+	"entgo.io/ent/dialect/sql"
+	"entgo.io/ent/dialect/sql/sqlgraph"
+	"entgo.io/ent/schema/field"
+	"github.com/Wei-Shaw/sub2api/ent/account"
+	"github.com/Wei-Shaw/sub2api/ent/predicate"
+)
+
+// AccountDelete is the builder for deleting a Account entity.
+type AccountDelete struct {
+	config
+	hooks    []Hook
+	mutation *AccountMutation
+}
+
+// Where appends a list predicates to the AccountDelete builder.
+func (_d *AccountDelete) Where(ps ...predicate.Account) *AccountDelete {
+	_d.mutation.Where(ps...)
+	return _d
+}
+
+// Exec executes the deletion query and returns how many vertices were deleted.
+func (_d *AccountDelete) 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 *AccountDelete) ExecX(ctx context.Context) int {
+	n, err := _d.Exec(ctx)
+	if err != nil {
+		panic(err)
+	}
+	return n
+}
+
+func (_d *AccountDelete) sqlExec(ctx context.Context) (int, error) {
+	_spec := sqlgraph.NewDeleteSpec(account.Table, sqlgraph.NewFieldSpec(account.FieldID, field.TypeInt64))
+	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
+}
+
+// AccountDeleteOne is the builder for deleting a single Account entity.
+type AccountDeleteOne struct {
+	_d *AccountDelete
+}
+
+// Where appends a list predicates to the AccountDelete builder.
+func (_d *AccountDeleteOne) Where(ps ...predicate.Account) *AccountDeleteOne {
+	_d._d.mutation.Where(ps...)
+	return _d
+}
+
+// Exec executes the deletion query.
+func (_d *AccountDeleteOne) Exec(ctx context.Context) error {
+	n, err := _d._d.Exec(ctx)
+	switch {
+	case err != nil:
+		return err
+	case n == 0:
+		return &NotFoundError{account.Label}
+	default:
+		return nil
+	}
+}
+
+// ExecX is like Exec, but panics if an error occurs.
+func (_d *AccountDeleteOne) ExecX(ctx context.Context) {
+	if err := _d.Exec(ctx); err != nil {
+		panic(err)
+	}
+}