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_coverage_remaining_gaps.py

@@ -620,6 +620,13 @@ def _set_minimal_provider_settings(mock_settings):
         "email_password": None,
         "email_use_tls": True,
         "email_sender": None,
+        "dest_email_host": None,
+        "dest_email_default_recipient": None,
+        "dest_email_port": 587,
+        "dest_email_username": None,
+        "dest_email_password": None,
+        "dest_email_use_tls": True,
+        "dest_email_sender": None,
         "ftp_host": None,
         "ftp_username": None,
         "ftp_password": None,