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)

backend/migrations/core/074_add_photo_cap.js

@@ -0,0 +1,13 @@
+const { addColumnIfNotExists } = require('../helpers');
+
+exports.up = async function(knex) {
+  console.log('Running migration: 074_add_photo_cap');
+  await addColumnIfNotExists(knex, 'events', 'photo_cap', (table) => {
+    table.integer('photo_cap').nullable().defaultTo(null);
+  });
+  console.log('Migration 074_add_photo_cap completed');
+};
+
+exports.down = async function(knex) {
+  console.log('Rollback: 074_add_photo_cap');
+};