Adds an optional `undated` query param to GET /api/documents/search and
/api/documents/ids, threaded through searchDocuments and findIdsForFilter
into the shared buildSearchSpec via undatedOnly(boolean). undated=true also
bypasses the pure-text RELEVANCE SQL shortcut, which skips buildSearchSpec
and would otherwise drop the predicate. The read GET stays unguarded
(WebMvc authz test pins 200 for an authenticated user, 401 unauthenticated).
A locking test proves the in-memory SENDER sort keeps undated letters under
their sender.
Refs #668
268c31a49b