refactor(backend): extract shared helper modules per architecture rules

- Add backend/ai/utils.py — parse_classification, parse_suggestions, strip_code_fences
  shared by all AI providers; removes duplicated private functions from
  anthropic_provider.py and openai_provider.py
- Add backend/deps/utils.py — get_client_ip, parse_uuid request-parsing helpers;
  removes local _ip() variants from admin.py, auth.py, shares.py, folders.py
- Add backend/storage/exceptions.py — canonical CloudConnectionError definition;
  all routers and backends import from here instead of redefining
- Move validate_password_strength to backend/services/auth.py; removes duplicated
  _validate_password_strength from admin.py and auth.py

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/tasks/document_tasks.py

@@ -76,12 +76,7 @@ async def _run(document_id: str) -> dict:
                 if user is None:
                     return {"document_id": document_id, "status": "missing_user"}
 
-                try:
-                    from storage.google_drive_backend import CloudConnectionError
-                except ImportError:
-                    class CloudConnectionError(Exception):  # type: ignore[no-redef]
-                        pass
-
+                from storage.exceptions import CloudConnectionError
                 try:
                     backend = await get_storage_backend_for_document(doc, user, session)
                     file_bytes = await backend.get_object(doc.object_key)