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/test/firehose_web/user_auth_test.exs

@@ -4,6 +4,7 @@ defmodule FirehoseWeb.UserAuthTest do
   alias Firehose.Accounts
   alias Firehose.Accounts.Scope
   alias FirehoseWeb.UserAuth
+  alias Phoenix.LiveView.Socket
 
   import Firehose.AccountsFixtures
 
@@ -27,6 +28,13 @@ defmodule FirehoseWeb.UserAuthTest do
       assert Accounts.get_user_by_session_token(token)
     end
 
+    test "stores a live socket id derived from the session token", %{conn: conn, user: user} do
+      conn = UserAuth.log_in_user(conn, user)
+      token = get_session(conn, :user_token)
+
+      assert get_session(conn, :live_socket_id) == "users_sessions:#{Base.url_encode64(token)}"
+    end
+
     test "clears everything previously stored in the session", %{conn: conn, user: user} do
       conn = conn |> put_session(:to_be_removed, "value") |> UserAuth.log_in_user(user)
       refute get_session(conn, :to_be_removed)
@@ -113,6 +121,21 @@ defmodule FirehoseWeb.UserAuthTest do
       refute Accounts.get_user_by_session_token(user_token)
     end
 
+    test "broadcasts a disconnect on the session's live socket id", %{conn: conn, user: user} do
+      logged_in_conn = UserAuth.log_in_user(conn, user)
+      token = get_session(logged_in_conn, :user_token)
+      live_socket_id = get_session(logged_in_conn, :live_socket_id)
+      FirehoseWeb.Endpoint.subscribe(live_socket_id)
+
+      conn
+      |> put_session(:user_token, token)
+      |> put_session(:live_socket_id, live_socket_id)
+      |> UserAuth.log_out_user()
+
+      assert_receive %Phoenix.Socket.Broadcast{event: "disconnect", topic: ^live_socket_id}
+      refute_receive %Phoenix.Socket.Broadcast{topic: ^live_socket_id}
+    end
+
     test "works even if user is already logged out", %{conn: conn} do
       conn = conn |> fetch_cookies() |> UserAuth.log_out_user()
       refute get_session(conn, :user_token)
@@ -183,6 +206,56 @@ defmodule FirehoseWeb.UserAuthTest do
       assert new_signed_token != signed_token
       assert max_age == @remember_me_cookie_max_age
     end
+
+    test "reissues the live socket id together with the token", %{conn: conn, user: user} do
+      logged_in_conn =
+        conn |> fetch_cookies() |> UserAuth.log_in_user(user, %{"remember_me" => "true"})
+
+      token = logged_in_conn.cookies[@remember_me_cookie]
+      %{value: signed_token} = logged_in_conn.resp_cookies[@remember_me_cookie]
+      old_live_socket_id = get_session(logged_in_conn, :live_socket_id)
+
+      offset_user_token(token, -10, :day)
+
+      conn =
+        conn
+        |> put_session(:user_token, token)
+        |> put_session(:user_remember_me, true)
+        |> put_session(:live_socket_id, old_live_socket_id)
+        |> put_req_cookie(@remember_me_cookie, signed_token)
+        |> UserAuth.fetch_current_scope_for_user([])
+
+      assert new_token = get_session(conn, :user_token)
+      refute new_token == token
+
+      assert get_session(conn, :live_socket_id) ==
+               "users_sessions:#{Base.url_encode64(new_token)}"
+
+      refute get_session(conn, :live_socket_id) == old_live_socket_id
+    end
+  end
+
+  describe "live view sockets" do
+    # A real websocket disconnect cannot be driven through Phoenix.LiveViewTest:
+    # the "disconnect" broadcast is handled by Phoenix.Socket.__info__/2 on the
+    # transport process (deps/phoenix/lib/phoenix/socket.ex:571), and the test
+    # harness (phoenix_live_view/test/client_proxy.ex) never subscribes to the
+    # socket id topic. What can be pinned is the contract the transport relies
+    # on: the value in the session is the topic Phoenix.LiveView.Socket.id/1
+    # returns for the session the websocket receives (string keys).
+    test "the id written to the session is the topic Socket.id/1 reads", %{conn: conn, user: user} do
+      conn = UserAuth.log_in_user(conn, user)
+      live_socket_id = get_session(conn, :live_socket_id)
+
+      socket_session =
+        Map.new(conn.private[:plug_session], fn {key, value} -> {to_string(key), value} end)
+
+      socket = %Socket{
+        private: %{connect_info: %{session: socket_session}}
+      }
+
+      assert Socket.id(socket) == live_socket_id
+    end
   end
 
   describe "require_sudo_mode/2" do