Remove knowledge graph feature

The knowledge graph feature (links graph) was not functioning correctly
and was slowing down the application. This commit removes it entirely:

- Delete knowledge_graph_urls.py, knowledge_graph_views.py, llm_client.py
- Delete knowledge_graph.html template
- Remove KnowledgeGraphSnapshot model and all llm_*/kg_* fields from
  SiteSettings (migration 0048)
- Remove build_knowledge_graph() and schedule_kg_build() from tasks.py
- Remove KG settings save logic and bulk delete action from views.py
- Remove knowledge_graph_urls include from links/urls.py
- Remove schedule_kg_build scheduler job from core/apps.py
- Remove KG snapshot job registry from links/apps.py
- Remove Knowledge Graph nav item from base.html
- Remove KG settings cards and llmSettingsHelper JS from settings.html

templates/base.html

@@ -191,16 +191,6 @@
                         </a>
 
 
-                        <a href="{% url 'knowledge-graph' %}" class="block px-4 py-2 text-sm text-gray-700 hover:bg-gray-100">
-                            <div class="flex items-center">
-                                <svg class="w-5 h-5 mr-2" fill="none" stroke="currentColor" viewBox="0 0 24 24">
-                                    <path stroke-linecap="round" stroke-linejoin="round" stroke-width="2"
-                                    d="M4 6a2 2 0 100-4 2 2 0 000 4zm16 0a2 2 0 100-4 2 2 0 000 4zM4 20a2 2 0 100-4 2 2 0 000 4zm16 0a2 2 0 100-4 2 2 0 000 4zm-8-8a2 2 0 100-4 2 2 0 000 4zM8.5 8.5l-3 3m13-3l-3 3m-7 0l3 3m1 0l3-3"/>
-                                </svg>
-                                {% trans "Knowledge Graph" %}
-                            </div>
-                        </a>
-
                         <a href="{% url 'jobs' %}" class="block px-4 py-2 text-sm text-gray-700 hover:bg-gray-100">
                             <div class="flex items-center">
                                 <svg class="w-5 h-5 mr-2" fill="none" stroke="currentColor" viewBox="0 0 24 24">