fix: resolve Invalid Date issue in email templates

- Fix date parsing for YYYY-MM-DD format to use local timezone
- Handle date strings properly to avoid timezone conversion issues
- Add validation to ensure dates are valid before formatting
- Update expiration date calculation to use consistent parsing

This fixes the "Invalid Date" display in gallery creation emails.

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

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

backend/src/routes/adminEvents.js

@@ -89,7 +89,14 @@ router.post('/', adminAuth, [
     const password_hash = await bcrypt.hash(password, getBcryptRounds());
     
     // Calculate expiration date (days after event date)
-    const expires_at = new Date(event_date);
+    // Parse YYYY-MM-DD format as local date to avoid timezone issues
+    let expires_at;
+    if (event_date.match(/^\d{4}-\d{2}-\d{2}$/)) {
+      const [year, month, day] = event_date.split('-').map(num => parseInt(num, 10));
+      expires_at = new Date(year, month - 1, day);
+    } else {
+      expires_at = new Date(event_date);
+    }
     expires_at.setDate(expires_at.getDate() + parseInt(expiration_days, 10));
     
     // Create folder structure