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/proxy/proxy.go

@@ -0,0 +1,152 @@
+// Code generated by ent, DO NOT EDIT.
+
+package proxy
+
+import (
+	"time"
+
+	"entgo.io/ent"
+	"entgo.io/ent/dialect/sql"
+)
+
+const (
+	// Label holds the string label denoting the proxy type in the database.
+	Label = "proxy"
+	// 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"
+	// FieldName holds the string denoting the name field in the database.
+	FieldName = "name"
+	// FieldProtocol holds the string denoting the protocol field in the database.
+	FieldProtocol = "protocol"
+	// FieldHost holds the string denoting the host field in the database.
+	FieldHost = "host"
+	// FieldPort holds the string denoting the port field in the database.
+	FieldPort = "port"
+	// FieldUsername holds the string denoting the username field in the database.
+	FieldUsername = "username"
+	// FieldPassword holds the string denoting the password field in the database.
+	FieldPassword = "password"
+	// FieldStatus holds the string denoting the status field in the database.
+	FieldStatus = "status"
+	// Table holds the table name of the proxy in the database.
+	Table = "proxies"
+)
+
+// Columns holds all SQL columns for proxy fields.
+var Columns = []string{
+	FieldID,
+	FieldCreatedAt,
+	FieldUpdatedAt,
+	FieldDeletedAt,
+	FieldName,
+	FieldProtocol,
+	FieldHost,
+	FieldPort,
+	FieldUsername,
+	FieldPassword,
+	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
+	// NameValidator is a validator for the "name" field. It is called by the builders before save.
+	NameValidator func(string) error
+	// ProtocolValidator is a validator for the "protocol" field. It is called by the builders before save.
+	ProtocolValidator func(string) error
+	// HostValidator is a validator for the "host" field. It is called by the builders before save.
+	HostValidator func(string) error
+	// UsernameValidator is a validator for the "username" field. It is called by the builders before save.
+	UsernameValidator func(string) error
+	// PasswordValidator is a validator for the "password" field. It is called by the builders before save.
+	PasswordValidator 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 Proxy 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()
+}
+
+// ByName orders the results by the name field.
+func ByName(opts ...sql.OrderTermOption) OrderOption {
+	return sql.OrderByField(FieldName, opts...).ToFunc()
+}
+
+// ByProtocol orders the results by the protocol field.
+func ByProtocol(opts ...sql.OrderTermOption) OrderOption {
+	return sql.OrderByField(FieldProtocol, opts...).ToFunc()
+}
+
+// ByHost orders the results by the host field.
+func ByHost(opts ...sql.OrderTermOption) OrderOption {
+	return sql.OrderByField(FieldHost, opts...).ToFunc()
+}
+
+// ByPort orders the results by the port field.
+func ByPort(opts ...sql.OrderTermOption) OrderOption {
+	return sql.OrderByField(FieldPort, opts...).ToFunc()
+}
+
+// ByUsername orders the results by the username field.
+func ByUsername(opts ...sql.OrderTermOption) OrderOption {
+	return sql.OrderByField(FieldUsername, opts...).ToFunc()
+}
+
+// ByPassword orders the results by the password field.
+func ByPassword(opts ...sql.OrderTermOption) OrderOption {
+	return sql.OrderByField(FieldPassword, opts...).ToFunc()
+}
+
+// ByStatus orders the results by the status field.
+func ByStatus(opts ...sql.OrderTermOption) OrderOption {
+	return sql.OrderByField(FieldStatus, opts...).ToFunc()
+}