Iterates the user's data dir and removes every case directory that
carries a .closed marker. Requires form field confirm=yes as a server-
side guard against accidental browser-history re-POSTs. Emits CasePurged
per successfully removed case; per-case failures are logged but do not
abort the sweep.
UI button is deferred to the show-closed step so the endpoint is only
reachable after an explicit navigation. Three integration tests cover
the happy path, missing-confirm, and coexistence with open cases.
Brummel
8173ff2f26