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

CLAUDE.md

@@ -494,6 +494,17 @@ BODY:
 <full email body here>
 ```
 
+## Task Preview Generation Rules
+
+- The frontend must **never** call LLM providers (OpenAI/Ollama/Llama) directly.
+- The frontend requests backend endpoints only.
+- The backend is responsible for:
+  - building the final prompt,
+  - calling the configured LLM endpoint,
+  - returning the generated message to the frontend.
+- Generated preview message history must be persisted in the backend database (not browser storage),
+  so history survives reloads and restarts.
+
 ---
 
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