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/web/templates/chat/_composer.html

@@ -50,6 +50,9 @@
       {% if not chat and current_model %}
       <input type="hidden" name="model_id" value="{{ current_model.model_id }}">
       {% endif %}
+      {% if not chat and starting_temporary %}
+      <input type="hidden" name="temporary" value="true">
+      {% endif %}
 
       <div class="composer__row">
         {% if can.get("files.upload") %}