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/setting/setting.go

@@ -0,0 +1,76 @@
+// Code generated by ent, DO NOT EDIT.
+
+package setting
+
+import (
+	"time"
+
+	"entgo.io/ent/dialect/sql"
+)
+
+const (
+	// Label holds the string label denoting the setting type in the database.
+	Label = "setting"
+	// FieldID holds the string denoting the id field in the database.
+	FieldID = "id"
+	// FieldKey holds the string denoting the key field in the database.
+	FieldKey = "key"
+	// FieldValue holds the string denoting the value field in the database.
+	FieldValue = "value"
+	// FieldUpdatedAt holds the string denoting the updated_at field in the database.
+	FieldUpdatedAt = "updated_at"
+	// Table holds the table name of the setting in the database.
+	Table = "settings"
+)
+
+// Columns holds all SQL columns for setting fields.
+var Columns = []string{
+	FieldID,
+	FieldKey,
+	FieldValue,
+	FieldUpdatedAt,
+}
+
+// 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 (
+	// KeyValidator is a validator for the "key" field. It is called by the builders before save.
+	KeyValidator func(string) error
+	// ValueValidator is a validator for the "value" field. It is called by the builders before save.
+	ValueValidator func(string) error
+	// 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
+)
+
+// OrderOption defines the ordering options for the Setting 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()
+}
+
+// ByKey orders the results by the key field.
+func ByKey(opts ...sql.OrderTermOption) OrderOption {
+	return sql.OrderByField(FieldKey, opts...).ToFunc()
+}
+
+// ByValue orders the results by the value field.
+func ByValue(opts ...sql.OrderTermOption) OrderOption {
+	return sql.OrderByField(FieldValue, opts...).ToFunc()
+}
+
+// ByUpdatedAt orders the results by the updated_at field.
+func ByUpdatedAt(opts ...sql.OrderTermOption) OrderOption {
+	return sql.OrderByField(FieldUpdatedAt, opts...).ToFunc()
+}