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/api/pages.py

@@ -83,6 +83,7 @@ def sidebar_context(db: DBSession, user: User) -> dict:
                 Chat.user_id == user.id,
                 Chat.folder_id.is_(None),
                 Chat.archived.is_(False),
+                Chat.temporary.is_(False),
             )
             .order_by(Chat.pinned.desc(), Chat.updated_at.desc())
         )
@@ -164,11 +165,15 @@ async def offline(request: Request) -> Response:
 
 
 @router.get("/chat")
-async def chat_index(request: Request, db: Db, user: RequiredUser, model: str = ""):
+async def chat_index(
+    request: Request, db: Db, user: RequiredUser, model: str = "", temporary: bool = False
+):
     """A composer with no chat behind it yet.
 
     `?model=` preselects one, which is how the pinned shortcuts work without
-    creating a row for a chat that may never be sent.
+    creating a row for a chat that may never be sent. `?temporary=1` is the
+    same idea for the temporary flag: it lives in the URL rather than in
+    JavaScript, so it survives a reload and can be bookmarked.
     """
     context = _chat_context(db, user, None)
 
@@ -195,6 +200,7 @@ async def chat_index(request: Request, db: Db, user: RequiredUser, model: str =
             "bodies": {},
             **context,
             "current_model": preselected,
+            "starting_temporary": temporary,
             **sidebar_context(db, user),
         },
     )