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/userallowedgroup/userallowedgroup.go

@@ -0,0 +1,113 @@
+// Code generated by ent, DO NOT EDIT.
+
+package userallowedgroup
+
+import (
+	"time"
+
+	"entgo.io/ent/dialect/sql"
+	"entgo.io/ent/dialect/sql/sqlgraph"
+)
+
+const (
+	// Label holds the string label denoting the userallowedgroup type in the database.
+	Label = "user_allowed_group"
+	// FieldUserID holds the string denoting the user_id field in the database.
+	FieldUserID = "user_id"
+	// FieldGroupID holds the string denoting the group_id field in the database.
+	FieldGroupID = "group_id"
+	// FieldCreatedAt holds the string denoting the created_at field in the database.
+	FieldCreatedAt = "created_at"
+	// EdgeUser holds the string denoting the user edge name in mutations.
+	EdgeUser = "user"
+	// EdgeGroup holds the string denoting the group edge name in mutations.
+	EdgeGroup = "group"
+	// UserFieldID holds the string denoting the ID field of the User.
+	UserFieldID = "id"
+	// GroupFieldID holds the string denoting the ID field of the Group.
+	GroupFieldID = "id"
+	// Table holds the table name of the userallowedgroup in the database.
+	Table = "user_allowed_groups"
+	// UserTable is the table that holds the user relation/edge.
+	UserTable = "user_allowed_groups"
+	// UserInverseTable is the table name for the User entity.
+	// It exists in this package in order to avoid circular dependency with the "user" package.
+	UserInverseTable = "users"
+	// UserColumn is the table column denoting the user relation/edge.
+	UserColumn = "user_id"
+	// GroupTable is the table that holds the group relation/edge.
+	GroupTable = "user_allowed_groups"
+	// GroupInverseTable is the table name for the Group entity.
+	// It exists in this package in order to avoid circular dependency with the "group" package.
+	GroupInverseTable = "groups"
+	// GroupColumn is the table column denoting the group relation/edge.
+	GroupColumn = "group_id"
+)
+
+// Columns holds all SQL columns for userallowedgroup fields.
+var Columns = []string{
+	FieldUserID,
+	FieldGroupID,
+	FieldCreatedAt,
+}
+
+// ValidColumn reports if the column name is valid (part of the table columns).
+func ValidColumn(column string) bool {
+	for i := range Columns {
+		if column == Columns[i] {
+			return true
+		}
+	}
+	return false
+}
+
+var (
+	// DefaultCreatedAt holds the default value on creation for the "created_at" field.
+	DefaultCreatedAt func() time.Time
+)
+
+// OrderOption defines the ordering options for the UserAllowedGroup queries.
+type OrderOption func(*sql.Selector)
+
+// ByUserID orders the results by the user_id field.
+func ByUserID(opts ...sql.OrderTermOption) OrderOption {
+	return sql.OrderByField(FieldUserID, opts...).ToFunc()
+}
+
+// ByGroupID orders the results by the group_id field.
+func ByGroupID(opts ...sql.OrderTermOption) OrderOption {
+	return sql.OrderByField(FieldGroupID, opts...).ToFunc()
+}
+
+// ByCreatedAt orders the results by the created_at field.
+func ByCreatedAt(opts ...sql.OrderTermOption) OrderOption {
+	return sql.OrderByField(FieldCreatedAt, opts...).ToFunc()
+}
+
+// ByUserField orders the results by user field.
+func ByUserField(field string, opts ...sql.OrderTermOption) OrderOption {
+	return func(s *sql.Selector) {
+		sqlgraph.OrderByNeighborTerms(s, newUserStep(), sql.OrderByField(field, opts...))
+	}
+}
+
+// ByGroupField orders the results by group field.
+func ByGroupField(field string, opts ...sql.OrderTermOption) OrderOption {
+	return func(s *sql.Selector) {
+		sqlgraph.OrderByNeighborTerms(s, newGroupStep(), sql.OrderByField(field, opts...))
+	}
+}
+func newUserStep() *sqlgraph.Step {
+	return sqlgraph.NewStep(
+		sqlgraph.From(Table, UserColumn),
+		sqlgraph.To(UserInverseTable, UserFieldID),
+		sqlgraph.Edge(sqlgraph.M2O, false, UserTable, UserColumn),
+	)
+}
+func newGroupStep() *sqlgraph.Step {
+	return sqlgraph.NewStep(
+		sqlgraph.From(Table, GroupColumn),
+		sqlgraph.To(GroupInverseTable, GroupFieldID),
+		sqlgraph.Edge(sqlgraph.M2O, false, GroupTable, GroupColumn),
+	)
+}