Fix #905: Handle invalid Hardcover tokens gracefully

Treat 401 Unauthorized responses from Hardcover API as missing/invalid
tokens instead of raising generic exceptions. This prevents error logs
when users don't have valid Hardcover API tokens and ensures proper
handling via the existing MissingHardcoverToken exception flow.

The fix allows users without Hardcover tokens to sync their Kobo devices
without generating ERROR-level log entries, logging an informational
message instead.

Author: crocodilestick

cps/services/hardcover.py

@@ -526,6 +526,9 @@ def execute(self, query, variables=None):
         try:
             response.raise_for_status()
         except requests.exceptions.HTTPError as e:
+            # 401 Unauthorized means invalid/missing token - treat as missing token
+            if response.status_code == 401:
+                raise MissingHardcoverToken("Invalid or expired Hardcover API token")
             raise Exception(f"HTTP error occurred: {e}")
         result = response.json()
         if "errors" in result: