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/account_group.go

@@ -0,0 +1,58 @@
+package schema
+
+import (
+	"time"
+
+	"entgo.io/ent"
+	"entgo.io/ent/dialect/entsql"
+	"entgo.io/ent/schema"
+	"entgo.io/ent/schema/edge"
+	"entgo.io/ent/schema/field"
+	"entgo.io/ent/schema/index"
+)
+
+// AccountGroup holds the edge schema definition for the account_groups relationship.
+// It stores extra fields (priority, created_at) and uses a composite primary key.
+type AccountGroup struct {
+	ent.Schema
+}
+
+func (AccountGroup) Annotations() []schema.Annotation {
+	return []schema.Annotation{
+		entsql.Annotation{Table: "account_groups"},
+		// Composite primary key: (account_id, group_id).
+		field.ID("account_id", "group_id"),
+	}
+}
+
+func (AccountGroup) Fields() []ent.Field {
+	return []ent.Field{
+		field.Int64("account_id"),
+		field.Int64("group_id"),
+		field.Int("priority").
+			Default(50),
+		field.Time("created_at").
+			Immutable().
+			Default(time.Now),
+	}
+}
+
+func (AccountGroup) Edges() []ent.Edge {
+	return []ent.Edge{
+		edge.To("account", Account.Type).
+			Unique().
+			Required().
+			Field("account_id"),
+		edge.To("group", Group.Type).
+			Unique().
+			Required().
+			Field("group_id"),
+	}
+}
+
+func (AccountGroup) Indexes() []ent.Index {
+	return []ent.Index{
+		index.Fields("group_id"),
+		index.Fields("priority"),
+	}
+}