Tags & categories gains a server-side tag search and sort (keyset pages,
cursor scoped to the query) and an "Untagged prompts" row whose Browse opens
the Library with tag_mode=untagged. Refine this view gets Tag match, saved
Library views keep it, and the routes echo the applied query so a stale
server is an error, never an unfiltered list. Recorded as D149.
Refs #444