Tier-gated RAG + Drive document sources (#42) (#54)

## Summary

Implements epic [#42](#42) (children #43–#53) and advances [#11](#11).

- **Entitlement:** `allows_rag` on plans (founders / backer / pro / business; not standard); exposed as `features.rag`
- **Gates:** document REST + WS `PromptType.RAG` use `assert_feature_allowed(..., "rag")`
- **Lifecycle:** dedupe ingest, delete vectors by `document_id`, honor `active`, fix document detail PATCH/DELETE
- **Workspaces:** auto-create default company workspace; fail-closed scoping
- **Drive:** personal + company Google/Microsoft connect (`link_drive` / `link_company_drive`), resource selection, sync, webhooks stubs, `sync_drive_connections` management command
- **Docs/env:** README + `.env*.example` updated

Companion FE: `chat_web_app` branch `feature/rag-epic-42-ui` (#81–#85).

## Test plan

- [x] `SKIP_RAG_INIT=1 uv run python manage.py test` (457 OK)
- [ ] Migrate finance `0004` + chat_backend `0028` on beta
- [ ] Verify Standard user: Documents API 403 + no RAG retrieval
- [ ] Verify Founders/Pro: upload + list + active toggle
- [ ] Connect Google/Microsoft Drive (incremental scopes) and Sync
- [ ] Company manager: `link_company_drive`; non-manager 403
- [ ] Run `manage.py sync_drive_connections`Reviewed-on: #54

llm_be/chat_backend/consumers_graph.py

@@ -294,8 +294,19 @@ async def generation_node(state: ChatState) -> ChatState:
              pass
 
     if prompt_type == PromptType.RAG:
-        service = AsyncRAGService()
         chat_user = state.get("chat_user")
+        if chat_user is not None:
+            try:
+                await enforce_feature_gate(chat_user, "rag")
+            except FeatureNotAllowed as exc:
+                return {
+                    "response_generator": {
+                        "type": "error",
+                        "code": exc.code,
+                        "content": exc.message,
+                    }
+                }
+        service = AsyncRAGService()
         workspace = await get_workspace(conversation_id, user=chat_user)
         generator = service.generate_response(messages, prompt_instance.message, workspace)
         return {"response_generator": generator}