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/api/documents.py

@@ -48,14 +48,7 @@ except ImportError:
     # Fallback for test environments where minio is not installed
     S3Error = Exception  # type: ignore[assignment,misc]
 
-try:
-    from storage.google_drive_backend import CloudConnectionError
-except ImportError:
-    # Fallback: define a stub so the except clause compiles even if google deps absent
-    class CloudConnectionError(Exception):  # type: ignore[no-redef]
-        def __init__(self, msg: str = "", *, reason: str = "") -> None:
-            super().__init__(msg)
-            self.reason = reason
+from storage.exceptions import CloudConnectionError
 
 # Valid cloud backend slugs (T-05-06-01: validated against allowlist, not user-supplied string)
 _CLOUD_PROVIDERS = frozenset({"google_drive", "onedrive", "nextcloud", "webdav"})