Replace the double-query pattern (get_folder_by_path + get_file_by_path)
across PROPFIND, HEAD, DELETE, MOVE, and COPY handlers with a single
UNION ALL query via PathResolverService.
PG Append node short-circuits on LIMIT 1: if the folder branch matches,
the file branch is never executed. Cuts WebDAV path resolution from
2 round-trips to 1 per request.
Also adds an exists() method using EXISTS subqueries for the Overwrite
header checks in MOVE/COPY (avoids constructing full DTOs).
Legacy double-query fallback retained when PathResolver is unavailable.
Dionisio