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/schema/user.go

@@ -0,0 +1,85 @@
+package schema
+
+import (
+	"github.com/Wei-Shaw/sub2api/ent/schema/mixins"
+	"github.com/Wei-Shaw/sub2api/internal/service"
+
+	"entgo.io/ent"
+	"entgo.io/ent/dialect"
+	"entgo.io/ent/dialect/entsql"
+	"entgo.io/ent/schema"
+	"entgo.io/ent/schema/edge"
+	"entgo.io/ent/schema/field"
+	"entgo.io/ent/schema/index"
+)
+
+// User holds the schema definition for the User entity.
+type User struct {
+	ent.Schema
+}
+
+func (User) Annotations() []schema.Annotation {
+	return []schema.Annotation{
+		entsql.Annotation{Table: "users"},
+	}
+}
+
+func (User) Mixin() []ent.Mixin {
+	return []ent.Mixin{
+		mixins.TimeMixin{},
+		mixins.SoftDeleteMixin{},
+	}
+}
+
+func (User) Fields() []ent.Field {
+	return []ent.Field{
+		field.String("email").
+			MaxLen(255).
+			NotEmpty().
+			Unique(),
+		field.String("password_hash").
+			MaxLen(255).
+			NotEmpty(),
+		field.String("role").
+			MaxLen(20).
+			Default(service.RoleUser),
+		field.Float("balance").
+			SchemaType(map[string]string{dialect.Postgres: "decimal(20,8)"}).
+			Default(0),
+		field.Int("concurrency").
+			Default(5),
+		field.String("status").
+			MaxLen(20).
+			Default(service.StatusActive),
+
+		// Optional profile fields (added later; default '' in DB migration)
+		field.String("username").
+			MaxLen(100).
+			Default(""),
+		field.String("wechat").
+			MaxLen(100).
+			Default(""),
+		field.String("notes").
+			SchemaType(map[string]string{dialect.Postgres: "text"}).
+			Default(""),
+	}
+}
+
+func (User) Edges() []ent.Edge {
+	return []ent.Edge{
+		edge.To("api_keys", ApiKey.Type),
+		edge.To("redeem_codes", RedeemCode.Type),
+		edge.To("subscriptions", UserSubscription.Type),
+		edge.To("assigned_subscriptions", UserSubscription.Type),
+		edge.To("allowed_groups", Group.Type).
+			Through("user_allowed_groups", UserAllowedGroup.Type),
+	}
+}
+
+func (User) Indexes() []ent.Index {
+	return []ent.Index{
+		index.Fields("email").Unique(),
+		index.Fields("status"),
+		index.Fields("deleted_at"),
+	}
+}