Cleaned up logic within ImageRepo

- Moved out extension check to ImageService as that seems more relevant.
- Updated models to use static-style references instead of facade to align with common modern usage within the app.
- Updated custom image_extension validation rule to use shared logic in image service.

app/Providers/CustomValidationServiceProvider.php

@@ -2,6 +2,7 @@
 
 namespace BookStack\Providers;
 
+use BookStack\Uploads\ImageService;
 use Illuminate\Support\Facades\Validator;
 use Illuminate\Support\ServiceProvider;
 
@@ -13,9 +14,8 @@ class CustomValidationServiceProvider extends ServiceProvider
     public function boot(): void
     {
         Validator::extend('image_extension', function ($attribute, $value, $parameters, $validator) {
-            $validImageExtensions = ['png', 'jpg', 'jpeg', 'gif', 'webp'];
-
-            return in_array(strtolower($value->getClientOriginalExtension()), $validImageExtensions);
+            $extension = strtolower($value->getClientOriginalExtension());
+            return ImageService::isExtensionSupported($extension);
         });
 
         Validator::extend('safe_url', function ($attribute, $value, $parameters, $validator) {