fix: derive inbox activity from mail, not thread.updated

Thread.updated is the root email's insert time. It never advances when a reply
arrives, despite the name and despite the schema describing threads as ordered
"most recently bumped". sr.ht returns updated seven seconds after root.date on
a thread carrying four replies:

    subject: "Re: test post"
    updated: 2026-07-16T03:44:55Z
    replies: 4
    root:    { date: 2026-07-15T22:44:48-05:00 }

The app fed that field into lastActivityAt everywhere, so a reply to an old
thread was invisible: it did not mark the thread unread, did not raise it in the
list, and did not update the timestamp beside it.

This was inert until the unread baseline landed, because an absent view record
returned unread unconditionally and the date was never compared to anything. The
baseline did not break unread; it was the first code to depend on that field
meaning what it says.

Use MailingList.emails instead, which is real reverse-chronological arrival
data, keyed by thread root id. The scan stops once it is older than the cutoff,
so a quiet list costs one page and a busy one costs only what is new. Threads
with nothing in the window keep the root timestamp and stay read, which they
already were. Failure returns empty activity rather than throwing: unread is a
decoration and should not take the thread list down with it.

Uses Email.received rather than Email.date — received is server-side and
non-null, date comes from the sender's header and is neither.

0 files changed, 0 insertions, 0 deletions