build log --follow follow-ups #251

Left from #250 (!462), each bounded today by the 10-minute queued limit, the reaper's deadline and the 8-per-account cap:

• Shutdown does not end open follows. hs.Shutdown does not cancel request contexts, so an open web follow holds the HTTP drain for the full 30 s and the page is cut mid-stream. Fix: hs.BaseContext cancelled on SIGTERM, and a server-wide stopping channel merged into sshd's done.
• Access is checked once in buildRef. A repository made private, or a viewer losing access, mid-follow keeps streaming until the build ends. Fix: re-run resolveRepo on poll ticks, or record it on the Threat-Model page.
• Each append wakes every follower and each reads substr(CAST(log AS BLOB), ?), which loads the whole value (up to 2 MB). Fix: wait 100–250 ms after a wake before reading, to coalesce appends.
• No unit test covers the sshd path from a client closing the session channel to Ctx.Done; for a quiet build it is the only thing that ends a follow.