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/db/models/chat.py

@@ -2,9 +2,10 @@
 
 from __future__ import annotations
 
+from datetime import datetime
 from typing import TYPE_CHECKING, Any
 
-from sqlalchemy import Boolean, ForeignKey, Integer, String, Text
+from sqlalchemy import Boolean, DateTime, ForeignKey, Integer, String, Text
 from sqlalchemy.orm import Mapped, mapped_column, relationship
 
 from lembas.db.base import Base, Timestamps, UUIDPrimaryKey
@@ -108,6 +109,19 @@ class Chat(UUIDPrimaryKey, Timestamps, Base):
     unread: Mapped[bool] = mapped_column(Boolean, default=False, nullable=False)
     unread_notified: Mapped[bool] = mapped_column(Boolean, default=False, nullable=False)
 
+    # --- Compaction ----------------------------------------------------------
+    # A summary of the turns up to `compacted_through_id`, sent in their place.
+    # The messages themselves are kept and still shown; they simply stop being
+    # part of the request. See services/compaction.py.
+    compact_summary: Mapped[str] = mapped_column(Text, default="")
+    # A plain id, deliberately not a ForeignKey: db/migrations.py compiles only
+    # the column type, so a REFERENCES clause would exist on a freshly created
+    # database and not on an upgraded one, and a constraint half the fleet has
+    # is worse than none. It is validated on every read instead -- the same
+    # reasoning `model_id` above carries.
+    compacted_through_id: Mapped[str | None] = mapped_column(String(32))
+    compacted_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True))
+
     folder: Mapped[Folder | None] = relationship(back_populates="chats")
     messages: Mapped[list[Message]] = relationship(
         back_populates="chat",