feat(backend): persist tasks and generated message history

- add EntityTask domain and CRUD API backed by PostgreSQL

- relate generated messages directly to tasks and delete on task removal

- move preview generation to backend Llama endpoint

- migrate frontend task APIs from localStorage to backend endpoints

- update tests and CLAUDE rules for backend-owned LLM/persistence

backend/src/main/resources/application.yml

@@ -48,6 +48,10 @@ openai:
   api-key: ${OPENAI_API_KEY}
   model: ${OPENAI_MODEL:gpt-4o}
 
+llama:
+  base-url: ${LLAMA_BASE_URL:http://localhost:11434}
+  model: ${LLAMA_MODEL:gemma3:4b}
+
 springdoc:
   swagger-ui:
     path: /swagger-ui.html