KEY DISCOVERY: 5xx errors are early signs of cookie expiration.
The backend returns 500 when cookie is expired but session not yet invalidated.
It takes several hours before it returns 401/403.
CHANGES:
1. On 5xx: throw AuthenticationError to trigger immediate re-login
2. Removed cookie validation logic (no longer needed)
3. Cache still preserved during re-login process
4. Re-login happens within same request, not on next request
This fixes the issue where expired cookies would cause 5xx errors
for hours before any re-login attempt was made.
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