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/Providers/MicropubServiceProvider.php

@@ -7,6 +7,7 @@ namespace App\Providers;
 use App\Services\Micropub\CardHandler;
 use App\Services\Micropub\EntryHandler;
 use App\Services\Micropub\MicropubHandlerRegistry;
+use App\Services\Micropub\UpdateHandler;
 use Illuminate\Support\ServiceProvider;
 
 class MicropubServiceProvider extends ServiceProvider
@@ -19,6 +20,7 @@ class MicropubServiceProvider extends ServiceProvider
             // Register handlers
             $registry->register('card', new CardHandler);
             $registry->register('entry', new EntryHandler);
+            $registry->register('update', new UpdateHandler);
 
             return $registry;
         });