Fix email service crash when Mailgun not configured

Add validation to check MAILGUN_API_URL has a valid protocol prefix
and MAILGUN_API_KEY is set before attempting to make HTTP request.
Returns False gracefully with warning log instead of crashing with
httpx.UnsupportedProtocol error.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/app/services/email_service.py

@@ -24,6 +24,17 @@ class EmailService:
         Returns:
             True if email was sent successfully, False otherwise
         """
+        # Validate Mailgun configuration
+        if not settings.MAILGUN_API_URL or not settings.MAILGUN_API_URL.startswith(
+            ("http://", "https://")
+        ):
+            print("Warning: MAILGUN_API_URL not configured or missing protocol")
+            return False
+
+        if not settings.MAILGUN_API_KEY:
+            print("Warning: MAILGUN_API_KEY not configured")
+            return False
+
         body = f"From: {user_name or 'Anonymous'}\nEmail: {user_email or 'Not provided'}\n\n{message}"
 
         async with httpx.AsyncClient() as client: