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/enttest/enttest.go

@@ -0,0 +1,84 @@
+// Code generated by ent, DO NOT EDIT.
+
+package enttest
+
+import (
+	"context"
+
+	"github.com/Wei-Shaw/sub2api/ent"
+	// required by schema hooks.
+	_ "github.com/Wei-Shaw/sub2api/ent/runtime"
+
+	"entgo.io/ent/dialect/sql/schema"
+	"github.com/Wei-Shaw/sub2api/ent/migrate"
+)
+
+type (
+	// TestingT is the interface that is shared between
+	// testing.T and testing.B and used by enttest.
+	TestingT interface {
+		FailNow()
+		Error(...any)
+	}
+
+	// Option configures client creation.
+	Option func(*options)
+
+	options struct {
+		opts        []ent.Option
+		migrateOpts []schema.MigrateOption
+	}
+)
+
+// WithOptions forwards options to client creation.
+func WithOptions(opts ...ent.Option) Option {
+	return func(o *options) {
+		o.opts = append(o.opts, opts...)
+	}
+}
+
+// WithMigrateOptions forwards options to auto migration.
+func WithMigrateOptions(opts ...schema.MigrateOption) Option {
+	return func(o *options) {
+		o.migrateOpts = append(o.migrateOpts, opts...)
+	}
+}
+
+func newOptions(opts []Option) *options {
+	o := &options{}
+	for _, opt := range opts {
+		opt(o)
+	}
+	return o
+}
+
+// Open calls ent.Open and auto-run migration.
+func Open(t TestingT, driverName, dataSourceName string, opts ...Option) *ent.Client {
+	o := newOptions(opts)
+	c, err := ent.Open(driverName, dataSourceName, o.opts...)
+	if err != nil {
+		t.Error(err)
+		t.FailNow()
+	}
+	migrateSchema(t, c, o)
+	return c
+}
+
+// NewClient calls ent.NewClient and auto-run migration.
+func NewClient(t TestingT, opts ...Option) *ent.Client {
+	o := newOptions(opts)
+	c := ent.NewClient(o.opts...)
+	migrateSchema(t, c, o)
+	return c
+}
+func migrateSchema(t TestingT, c *ent.Client, o *options) {
+	tables, err := schema.CopyTables(migrate.Tables)
+	if err != nil {
+		t.Error(err)
+		t.FailNow()
+	}
+	if err := migrate.Create(context.Background(), c.Schema, tables, o.migrateOpts...); err != nil {
+		t.Error(err)
+		t.FailNow()
+	}
+}