Compaction: a button, and automatically when the window fills

A long conversation eventually just stops working. Compaction summarises the
earlier turns and sends the summary in their place.

The messages are kept. They stay in the transcript behind a collapsed
divider and simply stop being part of the request, which is what makes the
button safe to press and automatic compaction safe to have at all: a summary
that came out badly is a bad turn, not a lost conversation.

Stored on the Chat, not as a synthetic Message. A synthetic row needs a
role -- `system` breaks the one-system-message rule the moment build_messages
emits it beside the harness, and user/assistant makes it a turn people can
edit, regenerate from and copy, indistinguishable from a real one in all
four places a bubble is rendered. Worse, "editing rewinds, it does not
branch" would silently delete it and leave no marker that compaction had
happened at all.

The summary goes out as a user turn and an assistant turn, not one. A
leading assistant breaks templates requiring the first non-system message to
be user; a lone leading user produces user, user whenever the kept history
starts on a user turn -- which it always does, because the cutoff lands on a
finished reply.

compacted_through_id is a plain id rather than a foreign key: migrations.py
compiles only the column type, so a REFERENCES clause would exist on a fresh
database and not on an upgraded one, and a constraint half the fleet has is
worse than none. cutoff_message validates it on every read instead, and a
rewind past the boundary clears it.

Compacting again summarises only the delta, with the previous summary
supplied to be subsumed. Re-summarising the whole chat each time grows
quadratically and eventually exceeds the window it is protecting.

Automatically at the top of _run, not in post_message: that route's contract
is to return immediately and leave the slow part to a resumable connection,
and it also means build_request is called once, after compaction, with no
second assembly path. The trigger is the last reply's recorded usage plus an
estimate of the new turn -- retrospective because true prompt_tokens are only
knowable after a response, plus the delta because otherwise fifty thousand
characters pasted into the composer overflow a window that read 90% last
turn. It never fires when the context length is unknown. It does fire on
estimated counts, which is safe here precisely because nothing is lost.

_maybe_compact never raises: a failure logs and sends the uncompacted
request. A `status` event says "Summarising earlier messages…" in the
meantime, because a silent multi-second pause before the first token is what
a hang looks like.

The wording is three fragments under Admin - Prompts. Clearing task.compact
turns compaction off entirely.

Also adds compaction.moment(): SQLite does not store the offset, so a row
loaded from disk is naive while one in the session's identity map keeps its
tzinfo, and comparing the two raises. Every comparison here is between
exactly those.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/lembas/web/static/css/chat.css

@@ -167,6 +167,45 @@
 .reasoning__summary::-webkit-details-marker { display: none; }
 .reasoning__summary:hover { color: var(--ink); background: var(--surface-hover); }
 
+/* --- Compacted turns -------------------------------------------------------
+   Summarised messages, kept and readable but out of the way. Collapsed by
+   default: the point of compacting was that they stopped mattering.
+*/
+.compacted {
+  border: 1px dashed var(--border-strong);
+  border-radius: var(--radius);
+  background: color-mix(in srgb, var(--surface) 50%, transparent);
+}
+.compacted__summary {
+  display: flex;
+  align-items: center;
+  gap: var(--sp-2);
+  padding: var(--sp-2) var(--sp-3);
+  cursor: pointer;
+  color: var(--ink-muted);
+  font-size: var(--text-sm);
+  list-style: none;
+  user-select: none;
+}
+.compacted__summary::-webkit-details-marker { display: none; }
+.compacted__summary:hover { color: var(--ink); }
+.compacted[open] .reasoning__chevron { transform: rotate(180deg); }
+
+.compacted__note {
+  margin: 0;
+  padding: 0 var(--sp-3);
+  font-size: var(--text-xs);
+  color: var(--ink-faint);
+  line-height: var(--leading-normal);
+}
+.compacted__body {
+  display: flex;
+  flex-direction: column;
+  gap: var(--sp-6);
+  padding: var(--sp-4) var(--sp-3);
+  opacity: 0.75;
+}
+
 /* --- Suggestions -----------------------------------------------------------
    Starting points on the empty screen. Cards rather than a list, because they
    are things to press.
@@ -333,6 +372,9 @@
 }
 
 /* --- Stop, notes and editing ---------------------------------------------- */
+.msg__status { font-size: var(--text-xs); color: var(--ink-faint); font-style: italic; }
+.msg__status:empty { display: none; }
+
 .msg__waiting {
   display: flex;
   align-items: center;