fix(chat): mark a page last when it reaches the beginning, cache topic names

[chrisnojima]

Service-side half of the thread-window work, split out of the pr this is based on so it can be reviewed on its own.

patchPaginationLast

A page that reaches message ID 1 - or the nukepoint an expunge recorded - is the last page whatever the pager says. Without this the client keeps asking for more of a conversation it has already seen the whole of, and the thread walks backwards through an expunged history one page at a time.

Topic-name cache

Resolving #channel tokens is charged per message: every body holding one read the inbox and then fetched the METADATA message of every channel in the team, so unboxing one page of a busy channel in a team with a few dozen channels cost thousands of single-message fetches. A short-TTL memory cache collapses the steady state.

Nothing invalidates the entry, so the TTL is the only bound on staleness and is deliberately short - the constant carries the argument, including what expiry does not heal, since a resolution is written into the message and persisted. A result missing channels is cached shorter still: the inbox read asks for every member status, so a team almost always carries channels the user has left or never joined and those fail on every pass, which makes "incomplete" the steady state rather than the exception.

Tests

convsource_patchpagination_test.go covers the last-page rule at both ends and that a settled page is left alone; teamchannelsource_topicnamecache_test.go covers the round trip, key separation, both-way copying, expiry on each of the two windows, and the clear paths. Run with go test ./chat/ -run 'TestPatchPagination|TestTopicName' - the rest of the package needs a local kbweb.