feat(example-images): add NSFW level setting endpoint

Add new POST endpoint `/api/lm/example-images/set-nsfw-level` to allow updating NSFW classification for individual example images. The endpoint supports both regular and custom images, validates required parameters, and updates the corresponding model metadata. This enables users to manually adjust NSFW ratings for better content filtering.

tests/routes/test_example_images_route_registrar_handlers.py

@@ -91,11 +91,16 @@ class StubImportUseCase:
 class StubProcessor:
     def __init__(self) -> None:
         self.delete_calls: list[web.Request] = []
+        self.nsfw_calls: list[web.Request] = []
 
     async def delete_custom_image(self, request: web.Request) -> web.Response:
         self.delete_calls.append(request)
         return web.json_response({"deleted": True})
 
+    async def set_example_image_nsfw_level(self, request: web.Request) -> web.Response:
+        self.nsfw_calls.append(request)
+        return web.json_response({"updated": True})
+
 
 class StubCleanupService:
     def __init__(self) -> None: