app: logout disconnects open LiveView sockets

`put_token_in_session/2` wrote only `:user_token`, so `log_out_user/1`'s
`if live_socket_id = get_session(conn, :live_socket_id)` was always false and
logging out left open LiveView sockets running. Write the socket topic
alongside the token, copying phx.gen.auth's format verbatim
("users_sessions:" <> Base.url_encode64(token)) so it matches both what
Phoenix.LiveView.Socket.id/1 reads and what log_out_user/1 reads. The reissue
path goes through the same function, so it follows the new token.

Tests (provider/reader agreement, both previously absent):
- log_in_user/3 stores the id derived from the session token
- fetch_current_scope_for_user/2 reissues the id with the new token
- log_out_user/1 broadcasts exactly one "disconnect" on that id
- the value written is the topic Socket.id/1 returns for the session the
  websocket receives (string keys)

Not tested end to end: a real disconnect needs the transport process, which
Phoenix.LiveViewTest does not start — Phoenix.Socket.__info__/2 handles the
broadcast (deps/phoenix/lib/phoenix/socket.ex:571) and
phoenix_live_view/test/client_proxy.ex never subscribes to the socket id topic.

Gate: make check + make verify green (app coverage 86.97%, blogex 75.16%,
crap sensors green).

Yak: logout-must-disconnect-existing-liveview-sockets-livesocketid-never-set-6fme

app/lib/firehose_web/user_auth.ex

@@ -171,9 +171,15 @@ defmodule FirehoseWeb.UserAuth do
   end
 
   defp put_token_in_session(conn, token) do
-    put_session(conn, :user_token, token)
+    conn
+    |> put_session(:user_token, token)
+    |> put_session(:live_socket_id, user_session_topic(token))
   end
 
+  # Must match what Phoenix.LiveView.Socket.id/1 reads out of the socket's
+  # connect_info session, so a "disconnect" broadcast reaches open sockets.
+  defp user_session_topic(token), do: "users_sessions:#{Base.url_encode64(token)}"
+
   @doc """
   Plug for routes that require sudo mode.
   """