Enhance pricing validation and update related components

- Added validation logic in `UpdatePricingSettingsCommandValidator` to ensure the annual price does not fall below the equivalent quarterly price, preventing potential pricing discrepancies.
- Updated `PricingSettings` model documentation to clarify that both pricing fields represent monthly rates, with calculations for total costs based on the number of months.
- Modified frontend components to reflect the new validation, including error messages when the annual price is cheaper than the quarterly price.
- Adjusted API documentation to accurately describe the pricing structure and validation rules for the pricing endpoints.

frontend/src/features/admin/pricing/PricingSettingsEditor.tsx

@@ -19,6 +19,13 @@ export function PricingSettingsEditor({ settings }: { settings: PricingSettingsD
     settings.pricePerConfigPerYear != null ? String(settings.pricePerConfigPerYear) : '',
   )
 
+  // Обе ставки — цена за конфиг в месяц; итог за год (ставка×12) не должен быть дешевле итога за
+  // квартал (ставка×3), иначе выгоднее купить "год" и не продлевать, чем платить за квартал.
+  const isYearCheaperThanQuarter =
+    pricePerConfigPerQuarter !== '' &&
+    pricePerConfigPerYear !== '' &&
+    Number(pricePerConfigPerYear) * 12 < Number(pricePerConfigPerQuarter) * 3
+
   const mutation = useMutation({
     mutationFn: () =>
       updatePricingSettings(
@@ -61,9 +68,12 @@ export function PricingSettingsEditor({ settings }: { settings: PricingSettingsD
           onChange={(e) => setPricePerConfigPerYear(e.target.value)}
         />
         <p className="text-xs text-muted-foreground">{t('admin.pricing.pricePerConfigPerYearHint')}</p>
+        {isYearCheaperThanQuarter && (
+          <p className="text-xs text-red-600">{t('admin.pricing.yearCheaperThanQuarter')}</p>
+        )}
       </div>
       <div>
-        <Button type="submit" disabled={mutation.isPending}>
+        <Button type="submit" disabled={mutation.isPending || isYearCheaperThanQuarter}>
           {t('admin.roles.save')}
         </Button>
       </div>