Implement discount tiers for pricing settings and enhance related functionalities

- Introduced a new `DiscountTierDto` to represent volume discount tiers, allowing roles with a config quota at or above specified thresholds to receive discounts on pricing.
- Updated the `PricingSettingsDto` to include a list of discount tiers, enhancing the pricing model to support more flexible pricing strategies.
- Modified the `GetPricingSettingsQueryHandler` and `UpdatePricingSettingsCommandHandler` to handle discount tiers, ensuring they are correctly retrieved and updated in the database.
- Enhanced validation in `UpdatePricingSettingsCommandValidator` to enforce uniqueness and progressive discount tiers, preventing invalid configurations.
- Updated frontend components to support the new discount tier functionality, including forms for adding and managing discount tiers in the admin interface.
- Revised API documentation to reflect the new discount tier features and their usage in pricing settings.

frontend/src/routes/admin/roles.tsx

@@ -8,6 +8,7 @@ import { Badge } from '@/shared/ui/badge'
 import { listRoles, deleteRole } from '@/features/admin/roles/api'
 import { RoleFormDialog } from '@/features/admin/roles/RoleFormDialog'
 import { getPricingSettings } from '@/features/admin/pricing/api'
+import { applyDiscount, resolveDiscountPercent } from '@/shared/lib/pricing'
 import type { RoleDto } from '@/shared/api/types'
 
 export const Route = createFileRoute('/admin/roles')({ component: AdminRolesPage })
@@ -20,9 +21,26 @@ function AdminRolesPage() {
   const { data, isLoading, isError, refetch } = useQuery({ queryKey: ['admin-roles'], queryFn: listRoles })
   const { data: pricing } = useQuery({ queryKey: ['admin-pricing'], queryFn: getPricingSettings })
 
-  // Ставки — цена за конфиг В МЕСЯЦ при данном тарифе; итог за период = ставка × месяцев × квота.
-  const totalPrice = (monthlyRate: number | null | undefined, maxConfigs: number, months: number) =>
-    monthlyRate == null || maxConfigs < 0 ? t('admin.roles.noPrice') : `${monthlyRate * months * maxConfigs} ₽`
+  // Ставки — цена за конфиг В МЕСЯЦ при данном тарифе; итог за период = ставка × месяцев × квота,
+  // затем скидка по лесенке (см. shared/lib/pricing) — за роль с большей квотой конфигов.
+  const priceCell = (monthlyRate: number | null | undefined, maxConfigs: number, months: number) => {
+    if (monthlyRate == null || maxConfigs < 0) return <span>{t('admin.roles.noPrice')}</span>
+
+    const original = monthlyRate * months * maxConfigs
+    const percent = resolveDiscountPercent(pricing?.discountTiers ?? [], maxConfigs)
+    if (percent <= 0) return <span>{original} ₽</span>
+
+    const discounted = applyDiscount(original, percent)
+    return (
+      <span className="flex flex-col">
+        <span className="text-xs text-muted-foreground line-through">{original} ₽</span>
+        <span className="flex items-center gap-1">
+          {discounted} ₽
+          <Badge variant="success">-{percent}%</Badge>
+        </span>
+      </span>
+    )
+  }
 
   const deleteMutation = useMutation({
     mutationFn: deleteRole,
@@ -74,9 +92,9 @@ function AdminRolesPage() {
                 </td>
                 <td className="py-2">{role.maxConfigs < 0 ? t('unlimited') : role.maxConfigs}</td>
                 <td className="py-2">{role.maxIpLimit < 0 ? t('unlimited') : role.maxIpLimit}</td>
-                <td className="py-2">{totalPrice(pricing?.pricePerConfigPerQuarter, role.maxConfigs, 3)}</td>
-                <td className="py-2">{totalPrice(pricing?.pricePerConfigPerHalfYear, role.maxConfigs, 6)}</td>
-                <td className="py-2">{totalPrice(pricing?.pricePerConfigPerYear, role.maxConfigs, 12)}</td>
+                <td className="py-2">{priceCell(pricing?.pricePerConfigPerQuarter, role.maxConfigs, 3)}</td>
+                <td className="py-2">{priceCell(pricing?.pricePerConfigPerHalfYear, role.maxConfigs, 6)}</td>
+                <td className="py-2">{priceCell(pricing?.pricePerConfigPerYear, role.maxConfigs, 12)}</td>
                 <td className="py-2 text-right">
                   <Button size="sm" variant="outline" onClick={() => setEditing(role)}>
                     {t('admin.roles.edit')}