Temporary chats

A clock in the top-right starts one. It is never listed in the sidebar and
is swept a day after the last thing said in it.

A real row rather than something held in the browser, because a reload, a
crash or a background tab all look identical from here -- "delete when you
navigate away" would lose conversations people meant to keep. The flag rides
in the URL (/chat?temporary=1) rather than in JavaScript, so it survives a
reload and can be bookmarked, and the composer carries it as a hidden field
beside model_id.

Keep clears the flag. Without a way out, a conversation that turns out to
matter is destroyed a day later with no recourse, and people would find that
out exactly once.

archived was filtered in three places and temporary mirrors all three, plus
Folder.visible_chats. It also skips the unread flag in _persist: there is no
sidebar row for the dot to land on, and the toast would name a chat nobody
can navigate to.

The sweep measures age from the newest message, not from the chat row.
created_at would destroy a conversation still in use at hour 23, and
updated_at does not move when a message is inserted -- onupdate fires on an
UPDATE of the chat, and adding a message is not one. It runs at startup
beside the existing upload sweep.

Deleting a chat cascades its rows but leaves the files on disk; only the
orphan sweep unlinks anything, and it looks only at uploads that were never
attached. files.remove_files_for_chats() closes that for the new sweep. The
same hole in delete_chat is pre-existing and left for its own change, which
can now call the same helper.

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

src/lembas/services/chat.py

@@ -3,9 +3,10 @@
 from __future__ import annotations
 
 import logging
+from datetime import UTC, datetime, timedelta
 from typing import Any
 
-from sqlalchemy import select
+from sqlalchemy import func, select
 from sqlalchemy.orm import Session as DBSession
 
 from lembas.db.models import (
@@ -32,6 +33,9 @@ FORWARDED_PARAMS = frozenset(
 
 MAX_TITLE_LENGTH = 60
 
+# How long a temporary chat survives after the last thing said in it.
+TEMPORARY_LIFETIME = timedelta(hours=24)
+
 
 def resolve_endpoint(db: DBSession, chat: Chat) -> tuple[Endpoint, str]:
     """Find the connection and model a chat should use.
@@ -387,8 +391,49 @@ def create_message(
     return message
 
 
+def sweep_temporary(db: DBSession, older_than: timedelta = TEMPORARY_LIFETIME) -> int:
+    """Delete temporary chats nobody has touched for a day.
+
+    Age is measured from the newest message rather than from the chat row's own
+    timestamps. `created_at` would destroy a conversation still in use at hour
+    23, and `updated_at` does not move when a message is inserted -- `onupdate`
+    fires on an UPDATE of the chat, and adding a message is not one.
+
+    Startup only, like files.sweep_orphans beside it. A server that runs for a
+    month sweeps once; that is the trade the existing sweep already makes, and a
+    scheduler is a whole new concern for a single-worker application.
+    """
+    cutoff = datetime.now(UTC) - older_than
+    newest = (
+        select(Message.chat_id, func.max(Message.created_at).label("last"))
+        .group_by(Message.chat_id)
+        .subquery()
+    )
+    stale = list(
+        db.scalars(
+            select(Chat)
+            .outerjoin(newest, newest.c.chat_id == Chat.id)
+            .where(
+                Chat.temporary.is_(True),
+                func.coalesce(newest.c.last, Chat.created_at) < cutoff,
+            )
+        )
+    )
+    if not stale:
+        return 0
+
+    files_service.remove_files_for_chats(db, [chat.id for chat in stale])
+    for chat in stale:
+        db.delete(chat)
+    db.commit()
+    log.info("swept %d temporary chat(s)", len(stale))
+    return len(stale)
+
+
 def user_chats(db: DBSession, user_id: str, *, folder_id: str | None = None) -> list[Chat]:
-    query = select(Chat).where(Chat.user_id == user_id, Chat.archived.is_(False))
+    query = select(Chat).where(
+        Chat.user_id == user_id, Chat.archived.is_(False), Chat.temporary.is_(False)
+    )
     if folder_id is not None:
         query = query.where(Chat.folder_id == folder_id)
     return list(db.scalars(query.order_by(Chat.pinned.desc(), Chat.updated_at.desc())))