Threads the caller's account-wide (used, available) storage figures through
PROPFIND for the plain-file WebDAV surface, resolved once per request via
StorageUsagePort::get_user_storage_info and reused for every folder entry
in the response. Unlimited accounts (quota <= 0) omit quota-available-bytes
entirely per RFC 4331 §3, rather than disclosing a sentinel value.
Properties are only advertised as known when the quota subsystem is enabled
and the lookup succeeds; otherwise they fall through to the standard 404
propstat.
M.Schmidt