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/redeemcode/redeemcode.go

@@ -0,0 +1,187 @@
+// Code generated by ent, DO NOT EDIT.
+
+package redeemcode
+
+import (
+	"time"
+
+	"entgo.io/ent/dialect/sql"
+	"entgo.io/ent/dialect/sql/sqlgraph"
+)
+
+const (
+	// Label holds the string label denoting the redeemcode type in the database.
+	Label = "redeem_code"
+	// FieldID holds the string denoting the id field in the database.
+	FieldID = "id"
+	// FieldCode holds the string denoting the code field in the database.
+	FieldCode = "code"
+	// FieldType holds the string denoting the type field in the database.
+	FieldType = "type"
+	// FieldValue holds the string denoting the value field in the database.
+	FieldValue = "value"
+	// FieldStatus holds the string denoting the status field in the database.
+	FieldStatus = "status"
+	// FieldUsedBy holds the string denoting the used_by field in the database.
+	FieldUsedBy = "used_by"
+	// FieldUsedAt holds the string denoting the used_at field in the database.
+	FieldUsedAt = "used_at"
+	// FieldNotes holds the string denoting the notes field in the database.
+	FieldNotes = "notes"
+	// FieldCreatedAt holds the string denoting the created_at field in the database.
+	FieldCreatedAt = "created_at"
+	// FieldGroupID holds the string denoting the group_id field in the database.
+	FieldGroupID = "group_id"
+	// FieldValidityDays holds the string denoting the validity_days field in the database.
+	FieldValidityDays = "validity_days"
+	// 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 redeemcode in the database.
+	Table = "redeem_codes"
+	// UserTable is the table that holds the user relation/edge.
+	UserTable = "redeem_codes"
+	// 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 = "used_by"
+	// GroupTable is the table that holds the group relation/edge.
+	GroupTable = "redeem_codes"
+	// 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 redeemcode fields.
+var Columns = []string{
+	FieldID,
+	FieldCode,
+	FieldType,
+	FieldValue,
+	FieldStatus,
+	FieldUsedBy,
+	FieldUsedAt,
+	FieldNotes,
+	FieldCreatedAt,
+	FieldGroupID,
+	FieldValidityDays,
+}
+
+// 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 (
+	// CodeValidator is a validator for the "code" field. It is called by the builders before save.
+	CodeValidator func(string) error
+	// DefaultType holds the default value on creation for the "type" field.
+	DefaultType string
+	// TypeValidator is a validator for the "type" field. It is called by the builders before save.
+	TypeValidator func(string) error
+	// DefaultValue holds the default value on creation for the "value" field.
+	DefaultValue float64
+	// 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
+	// DefaultCreatedAt holds the default value on creation for the "created_at" field.
+	DefaultCreatedAt func() time.Time
+	// DefaultValidityDays holds the default value on creation for the "validity_days" field.
+	DefaultValidityDays int
+)
+
+// OrderOption defines the ordering options for the RedeemCode 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()
+}
+
+// ByCode orders the results by the code field.
+func ByCode(opts ...sql.OrderTermOption) OrderOption {
+	return sql.OrderByField(FieldCode, opts...).ToFunc()
+}
+
+// ByType orders the results by the type field.
+func ByType(opts ...sql.OrderTermOption) OrderOption {
+	return sql.OrderByField(FieldType, opts...).ToFunc()
+}
+
+// ByValue orders the results by the value field.
+func ByValue(opts ...sql.OrderTermOption) OrderOption {
+	return sql.OrderByField(FieldValue, opts...).ToFunc()
+}
+
+// ByStatus orders the results by the status field.
+func ByStatus(opts ...sql.OrderTermOption) OrderOption {
+	return sql.OrderByField(FieldStatus, opts...).ToFunc()
+}
+
+// ByUsedBy orders the results by the used_by field.
+func ByUsedBy(opts ...sql.OrderTermOption) OrderOption {
+	return sql.OrderByField(FieldUsedBy, opts...).ToFunc()
+}
+
+// ByUsedAt orders the results by the used_at field.
+func ByUsedAt(opts ...sql.OrderTermOption) OrderOption {
+	return sql.OrderByField(FieldUsedAt, opts...).ToFunc()
+}
+
+// ByNotes orders the results by the notes field.
+func ByNotes(opts ...sql.OrderTermOption) OrderOption {
+	return sql.OrderByField(FieldNotes, opts...).ToFunc()
+}
+
+// ByCreatedAt orders the results by the created_at field.
+func ByCreatedAt(opts ...sql.OrderTermOption) OrderOption {
+	return sql.OrderByField(FieldCreatedAt, opts...).ToFunc()
+}
+
+// ByGroupID orders the results by the group_id field.
+func ByGroupID(opts ...sql.OrderTermOption) OrderOption {
+	return sql.OrderByField(FieldGroupID, opts...).ToFunc()
+}
+
+// ByValidityDays orders the results by the validity_days field.
+func ByValidityDays(opts ...sql.OrderTermOption) OrderOption {
+	return sql.OrderByField(FieldValidityDays, 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),
+	)
+}