Move CalDAV username/password from in-memory config to Secret Service
(gnome-keyring). CalDAVSyncProvider now loads credentials at sync time
and erases them on disconnect. OAuthSecretStore extended with
store/load/erase_caldav_credential methods using a separate libsecret
schema.
Add bounded token response parsing with refresh-token retention and a cancellable worker-thread libsecret adapter using versioned credential payloads.
Verify CRUD, isolation, cancellation, corruption, unavailable service, redaction, and cleanup against an isolated D-Bus and gnome-keyring Secret Service in normal, warning-as-error, and sanitizer profiles.
