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/accountgroup.go

@@ -0,0 +1,176 @@
+// Code generated by ent, DO NOT EDIT.
+
+package ent
+
+import (
+	"fmt"
+	"strings"
+	"time"
+
+	"entgo.io/ent"
+	"entgo.io/ent/dialect/sql"
+	"github.com/Wei-Shaw/sub2api/ent/account"
+	"github.com/Wei-Shaw/sub2api/ent/accountgroup"
+	"github.com/Wei-Shaw/sub2api/ent/group"
+)
+
+// AccountGroup is the model entity for the AccountGroup schema.
+type AccountGroup struct {
+	config `json:"-"`
+	// AccountID holds the value of the "account_id" field.
+	AccountID int64 `json:"account_id,omitempty"`
+	// GroupID holds the value of the "group_id" field.
+	GroupID int64 `json:"group_id,omitempty"`
+	// Priority holds the value of the "priority" field.
+	Priority int `json:"priority,omitempty"`
+	// CreatedAt holds the value of the "created_at" field.
+	CreatedAt time.Time `json:"created_at,omitempty"`
+	// Edges holds the relations/edges for other nodes in the graph.
+	// The values are being populated by the AccountGroupQuery when eager-loading is set.
+	Edges        AccountGroupEdges `json:"edges"`
+	selectValues sql.SelectValues
+}
+
+// AccountGroupEdges holds the relations/edges for other nodes in the graph.
+type AccountGroupEdges struct {
+	// Account holds the value of the account edge.
+	Account *Account `json:"account,omitempty"`
+	// Group holds the value of the group edge.
+	Group *Group `json:"group,omitempty"`
+	// loadedTypes holds the information for reporting if a
+	// type was loaded (or requested) in eager-loading or not.
+	loadedTypes [2]bool
+}
+
+// AccountOrErr returns the Account value or an error if the edge
+// was not loaded in eager-loading, or loaded but was not found.
+func (e AccountGroupEdges) AccountOrErr() (*Account, error) {
+	if e.Account != nil {
+		return e.Account, nil
+	} else if e.loadedTypes[0] {
+		return nil, &NotFoundError{label: account.Label}
+	}
+	return nil, &NotLoadedError{edge: "account"}
+}
+
+// GroupOrErr returns the Group value or an error if the edge
+// was not loaded in eager-loading, or loaded but was not found.
+func (e AccountGroupEdges) GroupOrErr() (*Group, error) {
+	if e.Group != nil {
+		return e.Group, nil
+	} else if e.loadedTypes[1] {
+		return nil, &NotFoundError{label: group.Label}
+	}
+	return nil, &NotLoadedError{edge: "group"}
+}
+
+// scanValues returns the types for scanning values from sql.Rows.
+func (*AccountGroup) scanValues(columns []string) ([]any, error) {
+	values := make([]any, len(columns))
+	for i := range columns {
+		switch columns[i] {
+		case accountgroup.FieldAccountID, accountgroup.FieldGroupID, accountgroup.FieldPriority:
+			values[i] = new(sql.NullInt64)
+		case accountgroup.FieldCreatedAt:
+			values[i] = new(sql.NullTime)
+		default:
+			values[i] = new(sql.UnknownType)
+		}
+	}
+	return values, nil
+}
+
+// assignValues assigns the values that were returned from sql.Rows (after scanning)
+// to the AccountGroup fields.
+func (_m *AccountGroup) assignValues(columns []string, values []any) error {
+	if m, n := len(values), len(columns); m < n {
+		return fmt.Errorf("mismatch number of scan values: %d != %d", m, n)
+	}
+	for i := range columns {
+		switch columns[i] {
+		case accountgroup.FieldAccountID:
+			if value, ok := values[i].(*sql.NullInt64); !ok {
+				return fmt.Errorf("unexpected type %T for field account_id", values[i])
+			} else if value.Valid {
+				_m.AccountID = value.Int64
+			}
+		case accountgroup.FieldGroupID:
+			if value, ok := values[i].(*sql.NullInt64); !ok {
+				return fmt.Errorf("unexpected type %T for field group_id", values[i])
+			} else if value.Valid {
+				_m.GroupID = value.Int64
+			}
+		case accountgroup.FieldPriority:
+			if value, ok := values[i].(*sql.NullInt64); !ok {
+				return fmt.Errorf("unexpected type %T for field priority", values[i])
+			} else if value.Valid {
+				_m.Priority = int(value.Int64)
+			}
+		case accountgroup.FieldCreatedAt:
+			if value, ok := values[i].(*sql.NullTime); !ok {
+				return fmt.Errorf("unexpected type %T for field created_at", values[i])
+			} else if value.Valid {
+				_m.CreatedAt = value.Time
+			}
+		default:
+			_m.selectValues.Set(columns[i], values[i])
+		}
+	}
+	return nil
+}
+
+// Value returns the ent.Value that was dynamically selected and assigned to the AccountGroup.
+// This includes values selected through modifiers, order, etc.
+func (_m *AccountGroup) Value(name string) (ent.Value, error) {
+	return _m.selectValues.Get(name)
+}
+
+// QueryAccount queries the "account" edge of the AccountGroup entity.
+func (_m *AccountGroup) QueryAccount() *AccountQuery {
+	return NewAccountGroupClient(_m.config).QueryAccount(_m)
+}
+
+// QueryGroup queries the "group" edge of the AccountGroup entity.
+func (_m *AccountGroup) QueryGroup() *GroupQuery {
+	return NewAccountGroupClient(_m.config).QueryGroup(_m)
+}
+
+// Update returns a builder for updating this AccountGroup.
+// Note that you need to call AccountGroup.Unwrap() before calling this method if this AccountGroup
+// was returned from a transaction, and the transaction was committed or rolled back.
+func (_m *AccountGroup) Update() *AccountGroupUpdateOne {
+	return NewAccountGroupClient(_m.config).UpdateOne(_m)
+}
+
+// Unwrap unwraps the AccountGroup entity that was returned from a transaction after it was closed,
+// so that all future queries will be executed through the driver which created the transaction.
+func (_m *AccountGroup) Unwrap() *AccountGroup {
+	_tx, ok := _m.config.driver.(*txDriver)
+	if !ok {
+		panic("ent: AccountGroup is not a transactional entity")
+	}
+	_m.config.driver = _tx.drv
+	return _m
+}
+
+// String implements the fmt.Stringer.
+func (_m *AccountGroup) String() string {
+	var builder strings.Builder
+	builder.WriteString("AccountGroup(")
+	builder.WriteString("account_id=")
+	builder.WriteString(fmt.Sprintf("%v", _m.AccountID))
+	builder.WriteString(", ")
+	builder.WriteString("group_id=")
+	builder.WriteString(fmt.Sprintf("%v", _m.GroupID))
+	builder.WriteString(", ")
+	builder.WriteString("priority=")
+	builder.WriteString(fmt.Sprintf("%v", _m.Priority))
+	builder.WriteString(", ")
+	builder.WriteString("created_at=")
+	builder.WriteString(_m.CreatedAt.Format(time.ANSIC))
+	builder.WriteByte(')')
+	return builder.String()
+}
+
+// AccountGroups is a parsable slice of AccountGroup.
+type AccountGroups []*AccountGroup