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/features/admin/pricing/PricingSettingsEditor.tsx

@@ -9,6 +9,8 @@ import { HttpError } from '@/shared/api/client'
 import type { PricingSettingsDto } from '@/shared/api/types'
 import { updatePricingSettings } from './api'
 
+type TierRow = { minConfigs: string; discountPercent: string }
+
 export function PricingSettingsEditor({ settings }: { settings: PricingSettingsDto }) {
   const { t } = useTranslation()
   const queryClient = useQueryClient()
@@ -21,6 +23,31 @@ export function PricingSettingsEditor({ settings }: { settings: PricingSettingsD
   const [pricePerConfigPerYear, setPricePerConfigPerYear] = useState(
     settings.pricePerConfigPerYear != null ? String(settings.pricePerConfigPerYear) : '',
   )
+  const [tiers, setTiers] = useState<TierRow[]>(
+    settings.discountTiers.map((t) => ({ minConfigs: String(t.minConfigs), discountPercent: String(t.discountPercent) })),
+  )
+
+  const updateTier = (index: number, patch: Partial<TierRow>) =>
+    setTiers((rows) => rows.map((row, i) => (i === index ? { ...row, ...patch } : row)))
+  const removeTier = (index: number) => setTiers((rows) => rows.filter((_, i) => i !== index))
+  const addTier = () => setTiers((rows) => [...rows, { minConfigs: '', discountPercent: '' }])
+
+  // Зеркалит бэкенд-инварианты UpdatePricingSettingsCommandValidator: пороги не повторяются, скидка
+  // на большем пороге не меньше скидки на меньшем (иначе лесенка не прогрессивная).
+  const tierErrors = (() => {
+    if (tiers.some((t) => t.minConfigs === '' || t.discountPercent === '')) return null
+    const parsed = tiers.map((t) => ({ minConfigs: Number(t.minConfigs), discountPercent: Number(t.discountPercent) }))
+    if (parsed.some((t) => !Number.isInteger(t.minConfigs) || t.minConfigs < 1)) return t('admin.pricing.discountTierInvalidMinConfigs')
+    if (parsed.some((t) => !Number.isInteger(t.discountPercent) || t.discountPercent < 1 || t.discountPercent > 99))
+      return t('admin.pricing.discountTierInvalidPercent')
+    if (new Set(parsed.map((t) => t.minConfigs)).size !== parsed.length) return t('admin.pricing.discountTiersDuplicate')
+    const sorted = [...parsed].sort((a, b) => a.minConfigs - b.minConfigs)
+    for (let i = 1; i < sorted.length; i++) {
+      if (sorted[i].discountPercent < sorted[i - 1].discountPercent) return t('admin.pricing.discountTiersNotProgressive')
+    }
+    return null
+  })()
+  const hasIncompleteTier = tiers.some((t) => t.minConfigs === '' || t.discountPercent === '')
 
   // Все ставки — цена за конфиг в месяц; итог за более длинный период (ставка × месяцев) не должен
   // быть дешевле итога за более короткий, иначе выгоднее купить длинный тариф и не продлевать.
@@ -48,6 +75,7 @@ export function PricingSettingsEditor({ settings }: { settings: PricingSettingsD
         pricePerConfigPerQuarter === '' ? null : Number(pricePerConfigPerQuarter),
         pricePerConfigPerHalfYear === '' ? null : Number(pricePerConfigPerHalfYear),
         pricePerConfigPerYear === '' ? null : Number(pricePerConfigPerYear),
+        tiers.map((t) => ({ minConfigs: Number(t.minConfigs), discountPercent: Number(t.discountPercent) })),
       ),
     onSuccess: async () => {
       toast.success(t('admin.pricing.updated'))
@@ -106,8 +134,40 @@ export function PricingSettingsEditor({ settings }: { settings: PricingSettingsD
           <p className="text-xs text-red-600">{t('admin.pricing.yearCheaperThanQuarter')}</p>
         )}
       </div>
+      <div className="flex flex-col gap-2">
+        <Label>{t('admin.pricing.discountTiersTitle')}</Label>
+        <p className="text-xs text-muted-foreground">{t('admin.pricing.discountTiersHint')}</p>
+        {tiers.map((tier, index) => (
+          <div key={index} className="flex items-center gap-2">
+            <Input
+              type="number"
+              min={1}
+              placeholder={t('admin.pricing.discountTierMinConfigs')}
+              value={tier.minConfigs}
+              onChange={(e) => updateTier(index, { minConfigs: e.target.value })}
+            />
+            <Input
+              type="number"
+              min={1}
+              max={99}
+              placeholder={t('admin.pricing.discountTierPercent')}
+              value={tier.discountPercent}
+              onChange={(e) => updateTier(index, { discountPercent: e.target.value })}
+            />
+            <Button type="button" variant="ghost" size="sm" onClick={() => removeTier(index)}>
+              {t('admin.pricing.removeDiscountTier')}
+            </Button>
+          </div>
+        ))}
+        {tierErrors && <p className="text-xs text-red-600">{tierErrors}</p>}
+        <div>
+          <Button type="button" variant="outline" size="sm" onClick={addTier}>
+            {t('admin.pricing.addDiscountTier')}
+          </Button>
+        </div>
+      </div>
       <div>
-        <Button type="submit" disabled={mutation.isPending || hasInvalidCombo}>
+        <Button type="submit" disabled={mutation.isPending || hasInvalidCombo || !!tierErrors || hasIncompleteTier}>
           {t('admin.roles.save')}
         </Button>
       </div>