Fix HTML content type handling with case-insensitive validation

- Normalize content_type to lowercase before processing to handle case-insensitive input (e.g., "HTML", "Html")
- Add validation in Validate() to ensure only valid content types are accepted
- Return descriptive error message if invalid content type is provided
- Fix condition in ToNotification() to properly detect and default content type
- Update SMTP notifier auto-detection to work correctly when content type is not explicitly set

Issues fixed:
1. Dynamically detecting content type now works correctly (was always defaulting to "text")
2. Client can now specify content type in request as "HTML", "html", or "Html" - all work
3. Invalid content types are rejected with clear error messages
4. Auto-detection still works if neither explicit type nor valid HTML markers are found

Example scenarios:
- No content_type field: auto-detects based on body (checks for <, <html, <!DOCTYPE, <p>, <div>, <br>)
- content_type: "html": sends as HTML with multipart/alternative
- content_type: "HTML": normalized to "html", sends as HTML
- content_type: "invalid": returns validation error
- content_type: "text": explicitly sends as plain text, skips auto-detection

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

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

internal/notifier/smtp.go

@@ -145,9 +145,9 @@ func (s *SMTPNotifier) buildMessage(notification *domain.Notification) string {
 	builder.WriteString(fmt.Sprintf("Subject: %s\r\n", notification.Subject))
 	builder.WriteString("MIME-Version: 1.0\r\n")
 
-	// Auto-detect HTML if content type not set
+	// Auto-detect HTML if content type not explicitly set to text
 	contentType := notification.ContentType
-	if contentType == "" {
+	if contentType == "" || contentType == "auto" {
 		contentType = detectContentType(notification.Body)
 	}