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") %}

src/lembas/web/templates/chat/index.html

@@ -25,6 +25,32 @@
       </h1>
 
       <div class="topbar__actions">
+        {#
+          A link, not a script: the flag lives in the URL, so it survives a
+          reload and can be bookmarked. On an existing temporary chat the same
+          corner explains what temporary means and offers the way out -- without
+          one, a conversation that turns out to matter is destroyed a day later
+          with no recourse.
+        #}
+        {% if chat and chat.temporary %}
+        <span class="badge badge--warning"
+              title="Not listed in the sidebar, and removed 24 hours after the last message.">
+          Temporary
+        </span>
+        <button class="btn btn--sm" type="button"
+                hx-post="/api/chats/{{ chat.id }}/keep" hx-swap="none"
+                title="Keep this chat and list it in the sidebar">
+          {{ icon("pin", "icon--sm") }} Keep
+        </button>
+        {% elif not chat and can.get("chat.create") %}
+        <a class="btn btn--icon {{ 'is-active' if starting_temporary }}"
+           href="{{ '/chat' if starting_temporary else '/chat?temporary=1' }}"
+           aria-label="Temporary chat"
+           title="{% if starting_temporary %}Starting a temporary chat. Click to go back to a normal one.{% else %}Start a temporary chat: not listed in the sidebar, and removed after a day.{% endif %}">
+          {{ icon("clock") }}
+        </a>
+        {% endif %}
+
         {% if models %}
           {% if can.get("chat.model_select") or not chat %}
             {% include "chat/_model_picker.html" %}