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>

app/utils/config_validator/validators.py

@@ -107,12 +107,12 @@ def validate_storage_configs() -> dict[str, list[str]]:
 
     issues["sftp"] = sftp_issues
 
-    # Validate Email sending
+    # Validate Email sending (destination-specific settings)
     email_issues = []
-    if not getattr(settings, "email_host", None):
-        email_issues.append("EMAIL_HOST is not configured")
-    if not getattr(settings, "email_default_recipient", None):
-        email_issues.append("EMAIL_DEFAULT_RECIPIENT is not configured")
+    if not getattr(settings, "dest_email_host", None):
+        email_issues.append("DEST_EMAIL_HOST is not configured")
+    if not getattr(settings, "dest_email_default_recipient", None):
+        email_issues.append("DEST_EMAIL_DEFAULT_RECIPIENT is not configured")
     issues["email"] = email_issues
 
     # Validate S3