Closes G4 / G5 / K5.
handle_move now resolves the destination once before the file/folder
dispatch and applies RFC 4918 §9.9.4:
- Overwrite: F on a collision → 412 Precondition Failed, source
untouched, destination untouched.
- Overwrite: T (or absent) on a collision → delete the existing
destination, then proceed → 204 No Content.
- No collision → 201 Created (unchanged).
The same destination lookup powers the 201-vs-204 status decision, so
adding the precondition guard adds zero extra DB hits on the happy
path.
handle_restore now catches the unique-index collision out of
restore_item and returns 412 instead of letting it bubble as 500.
Mirrors the G4 semantics for the trashbin surface (restore has no
Overwrite header so the refusal is unconditional; client resolves by
renaming the live file first).
Sabre/DAV's CorePlugin and our test pins agreed independently — NC
clients expect this exact behavior, and the new G5b/G5c positive-case
tests guard against a regression that hard-rejected every MOVE.
Edouard Vanbelle
5cb01b201d