feat: sort photos by capture date with configurable default sort (#283)

Add per-event default photo sort setting with 6 options:
- Upload Date (Newest/Oldest First)
- Date Taken (Newest/Oldest First) — uses EXIF captured_at
- Filename (A-Z / Z-A)

Backend:
- Migration 077 adds default_photo_sort column to events table
- Event create/update handlers accept and validate the setting
- Gallery info endpoint returns default_photo_sort for frontend

Frontend:
- "Date Taken" added to gallery sort dropdown (alongside Date, Name,
  Size, Rating)
- Gallery initializes with event's default sort instead of hardcoded
  "date"
- "Default Photo Sort" dropdown in event create and edit forms
- Photos without EXIF dates fall back to upload date

i18n: All 5 locales (EN, DE, NL, PT, RU) updated with sort labels.

Closes #283

backend/migrations/core/077_add_default_photo_sort.js

@@ -0,0 +1,17 @@
+exports.up = async function(knex) {
+  const hasColumn = await knex.schema.hasColumn('events', 'default_photo_sort');
+  if (!hasColumn) {
+    await knex.schema.alterTable('events', (table) => {
+      table.string('default_photo_sort', 50).defaultTo('upload_date_desc');
+    });
+  }
+};
+
+exports.down = async function(knex) {
+  const hasColumn = await knex.schema.hasColumn('events', 'default_photo_sort');
+  if (hasColumn) {
+    await knex.schema.alterTable('events', (table) => {
+      table.dropColumn('default_photo_sort');
+    });
+  }
+};