fix(email): create missing email template and decouple email destination settings

- Create app/templates/email/default.html (fixes 'default.html not found' error)
- Add DEST_EMAIL_* settings to app/config.py (decoupled from shared EMAIL_* settings)
- Update upload_to_email task to use dest_email_* settings exclusively
- Update _should_upload_to_email() to check dest_email_* settings
- Update config validator, providers, and settings_service for dest_email_*
- Update .env.demo and docs/ConfigurationGuide.md
- Update all tests to use dest_email_* settings where appropriate"

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

tests/test_send_to_all.py

@@ -120,10 +120,10 @@ class TestShouldUploadFunctions:
     @patch("app.tasks.send_to_all.settings")
     def test_should_upload_to_email_configured(self, mock_settings):
         """Test email upload check."""
-        mock_settings.email_host = "smtp.example.com"
-        mock_settings.email_username = "user"
-        mock_settings.email_password = "pass"
-        mock_settings.email_default_recipient = "recipient@example.com"
+        mock_settings.dest_email_host = "smtp.example.com"
+        mock_settings.dest_email_username = "user"
+        mock_settings.dest_email_password = "pass"
+        mock_settings.dest_email_default_recipient = "recipient@example.com"
 
         assert _should_upload_to_email() is True