feat: add resend creation email button and fix Handlebars template processing

- Fixed email template processing to properly handle Handlebars conditionals
  - Installed handlebars package
  - Updated processTemplate to use Handlebars.compile()
  - Now {{#if welcome_message}} blocks work correctly

- Added "Resend Creation Email" button to event details page
  - New endpoint: POST /api/admin/events/:id/resend-email
  - Button appears below "Reset Gallery Password"
  - Shows mail icon and includes success/error notifications
  - For security, resent emails show "(Aus Sicherheitsgründen nicht angezeigt)"
    instead of the actual password

- Added translations:
  - EN: "Resend Creation Email", success/error messages
  - DE: "Erstellungs-E-Mail erneut senden", success/error messages

This fixes the issue where Handlebars syntax was showing in emails and provides
a convenient way to resend the initial gallery creation email to hosts.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

backend/package.json

@@ -23,6 +23,7 @@
     "express-rate-limit": "^6.7.0",
     "express-validator": "^7.0.1",
     "form-data": "^4.0.3",
+    "handlebars": "^4.7.8",
     "helmet": "^7.0.0",
     "i18next": "^25.3.1",
     "i18next-browser-languagedetector": "^8.2.0",