Bring Micropub endpoint closer to the spec

- Detect form-encoded action=... requests, so form-encoded updates work
  and unhandled actions are no longer created as notes
- Default to h-entry when no type is given
- Read form-encoded photos from `photo` rather than `photos`
- Pass `category` through EntryData so bookmark tags are saved
- Accept single values for form-encoded multi-value properties
- Return 403 for insufficient_scope and 401 for invalid tokens
- Return 400 invalid_request for unsupported post types, update
  operations and models instead of reporting them as 500s

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

app/Services/Micropub/MicropubHandlerRegistry.php

@@ -51,7 +51,7 @@ class MicropubHandlerRegistry
     public function getHandler(string $type): MicropubHandlerInterface
     {
         if (! isset($this->handlers[$type])) {
-            throw new MicropubHandlerException("No handler registered for '{$type}'");
+            throw new MicropubHandlerException("Unsupported Micropub request type '{$type}'");
         }
 
         return $this->handlers[$type];