feat: add photo cap per event and Portuguese (pt-BR) locale

- Add photo_cap column to events table (migration 074) to limit photos per event
- Enforce photo cap in upload route, returning 400 when limit exceeded
- Pass photo_cap through all event CRUD routes and frontend forms
- Add complete Portuguese (pt-BR) translation (2300+ strings)
- Register pt locale in i18n config, language selector, date formatting
- Add photoCap/photoCapHelp translation keys to all locale files (en, de, ru, pt)

frontend/src/features/settings/tabs/GeneralTab.tsx

@@ -246,6 +246,7 @@ export const GeneralTab: React.FC<GeneralTabProps> = ({
             >
               <option value="en">English</option>
               <option value="de">Deutsch</option>
+              <option value="pt">Português (Brasil)</option>
             </select>
             <p className="text-xs text-neutral-500 dark:text-neutral-400 mt-1">
               {t('settings.general.defaultLanguageHelp')}