Personal Drive/RAG without a company (#55) (#56)

## Summary
- Closes [#55](#55) (related [#46](#46))
- **Personal** Google Drive / OneDrive works for users **without** a company (personal RAG)
- **Company** Drive still requires company + manager
- Schema: nullable `DriveConnection.company`, personal `DocumentWorkspace.user`, ownership check constraints + conditional uniques
- Runtime: `ensure_personal_workspace`, personal sync → personal WS, chat/document APIs fall back to personal WS when `company_id` is null
- Supersedes the interim "reject with `no_company`" approach (wrong for personal connect)

## Test plan
- [x] OAuth: personal Drive callback with `user.company=NULL` succeeds (`company_id=NULL` on connection)
- [x] OAuth: company Drive still returns `no_company` / `forbidden` appropriately
- [x] Chat tenant scope creates personal workspace for solo users
- [x] Drive sync + document view suites (`91` related tests)
- [ ] Manual: solo entitled user connects Google Drive → success, sync lands in personal workspace
- [ ] Manual: company manager company Drive still works
- [ ] Migrate prod/staging with `0029_personal_drive_rag_without_company`Reviewed-on: #56

llm_be/chat_backend/admin.py

@@ -207,6 +207,7 @@ class DocumentWorkspaceAdmin(admin.ModelAdmin):
     list_display = (
         "name",
         "company",
+        "user",
     )