Drive sync progress + documents list API + prompt heatmap (#59, #60, #94) (#61)

## Summary
- **#59** — Persist and expose Drive sync progress (`sync_total`, `sync_processed`, `sync_added`, `sync_updated`, `sync_failed`) during sync so the FE can show a progress bar.
- **#60** — Paginate/search/sort `GET /documents/` with `workspace=personal|company` scope isolation.
- **#94 (API)** — Add `GET /analytics/user_prompt_heatmap/?tz=` weekday × hour bins for user-entered prompts.

## Test plan
- [x] `manage.py test` drive sync / documents / analytics / drive_tasks suites
- [ ] Manual: trigger Drive sync and poll connections → progress fields update while pending
- [ ] Manual: `GET /documents/?workspace=personal&page=1&search=…&ordering=name`
- [ ] Deploy FE companion PR for #92/#93/#94 UI

Closes #59
Closes #60Reviewed-on: #61

llm_be/chat_backend/serializers.py

@@ -298,6 +298,11 @@ class DriveConnectionSerializer(serializers.ModelSerializer):
             "last_sync_at",
             "last_sync_status",
             "last_sync_error",
+            "sync_total",
+            "sync_processed",
+            "sync_added",
+            "sync_updated",
+            "sync_failed",
             "is_active",
             "created",
         ]