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/db/models/chat.py

@@ -58,7 +58,7 @@ class Folder(UUIDPrimaryKey, Timestamps, Base):
 
         Ordered like the unfiled list: pinned first, then most recently touched.
         """
-        kept = [chat for chat in self.chats if not chat.archived]
+        kept = [chat for chat in self.chats if not chat.archived and not chat.temporary]
         kept.sort(key=lambda chat: chat.updated_at, reverse=True)
         kept.sort(key=lambda chat: not chat.pinned)
         return kept
@@ -95,6 +95,13 @@ class Chat(UUIDPrimaryKey, Timestamps, Base):
     pinned: Mapped[bool] = mapped_column(Boolean, default=False, nullable=False)
     archived: Mapped[bool] = mapped_column(Boolean, default=False, nullable=False)
 
+    # Never listed in the sidebar, and swept a day after the last thing said in
+    # it. A real row rather than something held in the browser, so a reload or a
+    # dropped connection does not lose the conversation -- and `Keep` clears the
+    # flag, because a temporary chat that turns out to matter must have a way
+    # out. See services/chat.py:sweep_temporary.
+    temporary: Mapped[bool] = mapped_column(Boolean, default=False, nullable=False)
+
     # A reply landed while nobody was watching this chat. Cleared when the chat
     # is next opened. `unread_notified` stops the same arrival being announced
     # on every poll.