feat(计费): 支持账号计费倍率快照与统计展示

- 新增 accounts.rate_multiplier（默认 1.0，允许 0）
- 使用 usage_logs.account_rate_multiplier 记录倍率快照，避免历史回算
- 统计/导出/管理端展示账号口径费用（total_cost * account_rate_multiplier）

backend/ent/account.go

@@ -43,6 +43,8 @@ type Account struct {
 	Concurrency int `json:"concurrency,omitempty"`
 	// Priority holds the value of the "priority" field.
 	Priority int `json:"priority,omitempty"`
+	// RateMultiplier holds the value of the "rate_multiplier" field.
+	RateMultiplier float64 `json:"rate_multiplier,omitempty"`
 	// Status holds the value of the "status" field.
 	Status string `json:"status,omitempty"`
 	// ErrorMessage holds the value of the "error_message" field.
@@ -135,6 +137,8 @@ func (*Account) scanValues(columns []string) ([]any, error) {
 			values[i] = new([]byte)
 		case account.FieldAutoPauseOnExpired, account.FieldSchedulable:
 			values[i] = new(sql.NullBool)
+		case account.FieldRateMultiplier:
+			values[i] = new(sql.NullFloat64)
 		case account.FieldID, account.FieldProxyID, account.FieldConcurrency, account.FieldPriority:
 			values[i] = new(sql.NullInt64)
 		case account.FieldName, account.FieldNotes, account.FieldPlatform, account.FieldType, account.FieldStatus, account.FieldErrorMessage, account.FieldSessionWindowStatus:
@@ -241,6 +245,12 @@ func (_m *Account) assignValues(columns []string, values []any) error {
 			} else if value.Valid {
 				_m.Priority = int(value.Int64)
 			}
+		case account.FieldRateMultiplier:
+			if value, ok := values[i].(*sql.NullFloat64); !ok {
+				return fmt.Errorf("unexpected type %T for field rate_multiplier", values[i])
+			} else if value.Valid {
+				_m.RateMultiplier = value.Float64
+			}
 		case account.FieldStatus:
 			if value, ok := values[i].(*sql.NullString); !ok {
 				return fmt.Errorf("unexpected type %T for field status", values[i])
@@ -420,6 +430,9 @@ func (_m *Account) String() string {
 	builder.WriteString("priority=")
 	builder.WriteString(fmt.Sprintf("%v", _m.Priority))
 	builder.WriteString(", ")
+	builder.WriteString("rate_multiplier=")
+	builder.WriteString(fmt.Sprintf("%v", _m.RateMultiplier))
+	builder.WriteString(", ")
 	builder.WriteString("status=")
 	builder.WriteString(_m.Status)
 	builder.WriteString(", ")