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