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/apikey/apikey.go

@@ -0,0 +1,177 @@
+// Code generated by ent, DO NOT EDIT.
+
+package apikey
+
+import (
+	"time"
+
+	"entgo.io/ent"
+	"entgo.io/ent/dialect/sql"
+	"entgo.io/ent/dialect/sql/sqlgraph"
+)
+
+const (
+	// Label holds the string label denoting the apikey type in the database.
+	Label = "api_key"
+	// FieldID holds the string denoting the id field in the database.
+	FieldID = "id"
+	// FieldCreatedAt holds the string denoting the created_at field in the database.
+	FieldCreatedAt = "created_at"
+	// FieldUpdatedAt holds the string denoting the updated_at field in the database.
+	FieldUpdatedAt = "updated_at"
+	// FieldDeletedAt holds the string denoting the deleted_at field in the database.
+	FieldDeletedAt = "deleted_at"
+	// FieldUserID holds the string denoting the user_id field in the database.
+	FieldUserID = "user_id"
+	// FieldKey holds the string denoting the key field in the database.
+	FieldKey = "key"
+	// FieldName holds the string denoting the name field in the database.
+	FieldName = "name"
+	// FieldGroupID holds the string denoting the group_id field in the database.
+	FieldGroupID = "group_id"
+	// FieldStatus holds the string denoting the status field in the database.
+	FieldStatus = "status"
+	// 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"
+	// Table holds the table name of the apikey in the database.
+	Table = "api_keys"
+	// UserTable is the table that holds the user relation/edge.
+	UserTable = "api_keys"
+	// 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 = "api_keys"
+	// 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 apikey fields.
+var Columns = []string{
+	FieldID,
+	FieldCreatedAt,
+	FieldUpdatedAt,
+	FieldDeletedAt,
+	FieldUserID,
+	FieldKey,
+	FieldName,
+	FieldGroupID,
+	FieldStatus,
+}
+
+// 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
+}
+
+// Note that the variables below are initialized by the runtime
+// package on the initialization of the application. Therefore,
+// it should be imported in the main as follows:
+//
+//	import _ "github.com/Wei-Shaw/sub2api/ent/runtime"
+var (
+	Hooks        [1]ent.Hook
+	Interceptors [1]ent.Interceptor
+	// DefaultCreatedAt holds the default value on creation for the "created_at" field.
+	DefaultCreatedAt func() time.Time
+	// DefaultUpdatedAt holds the default value on creation for the "updated_at" field.
+	DefaultUpdatedAt func() time.Time
+	// UpdateDefaultUpdatedAt holds the default value on update for the "updated_at" field.
+	UpdateDefaultUpdatedAt func() time.Time
+	// KeyValidator is a validator for the "key" field. It is called by the builders before save.
+	KeyValidator func(string) error
+	// NameValidator is a validator for the "name" field. It is called by the builders before save.
+	NameValidator func(string) error
+	// DefaultStatus holds the default value on creation for the "status" field.
+	DefaultStatus string
+	// StatusValidator is a validator for the "status" field. It is called by the builders before save.
+	StatusValidator func(string) error
+)
+
+// OrderOption defines the ordering options for the ApiKey queries.
+type OrderOption func(*sql.Selector)
+
+// ByID orders the results by the id field.
+func ByID(opts ...sql.OrderTermOption) OrderOption {
+	return sql.OrderByField(FieldID, opts...).ToFunc()
+}
+
+// ByCreatedAt orders the results by the created_at field.
+func ByCreatedAt(opts ...sql.OrderTermOption) OrderOption {
+	return sql.OrderByField(FieldCreatedAt, opts...).ToFunc()
+}
+
+// ByUpdatedAt orders the results by the updated_at field.
+func ByUpdatedAt(opts ...sql.OrderTermOption) OrderOption {
+	return sql.OrderByField(FieldUpdatedAt, opts...).ToFunc()
+}
+
+// ByDeletedAt orders the results by the deleted_at field.
+func ByDeletedAt(opts ...sql.OrderTermOption) OrderOption {
+	return sql.OrderByField(FieldDeletedAt, opts...).ToFunc()
+}
+
+// ByUserID orders the results by the user_id field.
+func ByUserID(opts ...sql.OrderTermOption) OrderOption {
+	return sql.OrderByField(FieldUserID, opts...).ToFunc()
+}
+
+// ByKey orders the results by the key field.
+func ByKey(opts ...sql.OrderTermOption) OrderOption {
+	return sql.OrderByField(FieldKey, opts...).ToFunc()
+}
+
+// ByName orders the results by the name field.
+func ByName(opts ...sql.OrderTermOption) OrderOption {
+	return sql.OrderByField(FieldName, opts...).ToFunc()
+}
+
+// ByGroupID orders the results by the group_id field.
+func ByGroupID(opts ...sql.OrderTermOption) OrderOption {
+	return sql.OrderByField(FieldGroupID, opts...).ToFunc()
+}
+
+// ByStatus orders the results by the status field.
+func ByStatus(opts ...sql.OrderTermOption) OrderOption {
+	return sql.OrderByField(FieldStatus, 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, FieldID),
+		sqlgraph.To(UserInverseTable, FieldID),
+		sqlgraph.Edge(sqlgraph.M2O, true, UserTable, UserColumn),
+	)
+}
+func newGroupStep() *sqlgraph.Step {
+	return sqlgraph.NewStep(
+		sqlgraph.From(Table, FieldID),
+		sqlgraph.To(GroupInverseTable, FieldID),
+		sqlgraph.Edge(sqlgraph.M2O, true, GroupTable, GroupColumn),
+	)
+}