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/storage/exceptions.py

@@ -0,0 +1,19 @@
+"""Storage exception types — import from here, never redefine elsewhere."""
+from __future__ import annotations
+
+
+class CloudConnectionError(Exception):
+    """Raised when a cloud provider signals a non-retryable connection problem.
+
+    Attributes:
+        reason: "token_expired"  — access token expired; API layer can refresh and retry.
+                "invalid_grant" — refresh token revoked; user must reconnect.
+
+    The backend never updates the DB. The API layer (_call_cloud_op in cloud.py)
+    catches this exception, performs the DB state transition, and decides whether
+    to retry or surface a 503 to the client (B2 design, D-05/D-06).
+    """
+
+    def __init__(self, msg: str = "", *, reason: str = "") -> None:
+        super().__init__(msg)
+        self.reason = reason  # "token_expired" | "invalid_grant"