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/files.py

@@ -435,6 +435,26 @@ def claim(db: DBSession, *, ids: list[str], user_id: str, message_id: str) -> li
     return pending
 
 
+def remove_files_for_chats(db: DBSession, chat_ids: list[str]) -> int:
+    """Unlink the files belonging to these chats' attachments.
+
+    Deleting a Chat cascades to its Message and Attachment *rows* but leaves the
+    files on disk -- only `sweep_orphans` unlinks anything, and it only looks at
+    uploads that were never attached. Anything that deletes chats has to call
+    this first, while the rows still say which files to remove.
+    """
+    if not chat_ids:
+        return 0
+
+    removed = 0
+    for attachment in db.scalars(select(Attachment).where(Attachment.chat_id.in_(chat_ids))):
+        path = stored_path(attachment.stored_name)
+        if path is not None and path.exists():
+            path.unlink(missing_ok=True)
+            removed += 1
+    return removed
+
+
 def sweep_orphans(db: DBSession, older_than: timedelta = ORPHAN_AGE) -> int:
     """Delete uploads that were never attached to a message.