Introduce typed DTOs and sub-namespaces for Micropub handlers

Each handler now declares the data class it needs via dataClass(). The
controller builds the appropriate typed DTO from the raw request array
before calling handle(), giving handlers typed property access instead
of raw array lookups.

Handlers moved to App\Services\Micropub\Handlers, data objects to
App\Services\Micropub\Data. MicropubHandlerRegistry and the interface
are documented with flow diagrams and guidance for adding new types.

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

app/Http/Controllers/MicropubController.php

@@ -45,7 +45,9 @@ class MicropubController extends Controller
 
         try {
             $handler = $this->handlerRegistry->getHandler($type);
-            $result = $handler->handle($request->getMicropubData());
+            $dataClass = $handler->dataClass();
+            $data = $dataClass::fromArray($request->getMicropubData());
+            $result = $handler->handle($data);
 
             if ($result['response'] === 'updated') {
                 return response()->json([