Implement micropub update support with clean exception-based error handling

- Add MicropubUnsupportedModelException for non-note update attempts
- Refactor UpdateHandler to throw exceptions instead of returning JsonResponse objects; extract applySyndication() to DRY up duplicated syndication URL mapping
- Fix MicropubController: InvalidTokenScopeException now returns 401 + insufficient_scope; add catches for ModelNotFoundException (404) and MicropubUnsupportedModelException (500); updates return 200 not 201
- Slim MicropubRequest.normalizeMicropubJson() to branch on action type, keeping update and create fields cleanly separated
- Update tests to match corrected status codes and error keys; remove markTestSkipped() from all update tests

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/Exceptions/MicropubUnsupportedModelException.php

@@ -0,0 +1,7 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Exceptions;
+
+class MicropubUnsupportedModelException extends \Exception {}