Link devices to sessions for automatic cleanup on logout

When a session is destroyed (user logs out), all devices registered
to that session are automatically destroyed, preventing push
notifications from being sent to logged-out devices.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

saas/test/models/session/devices_test.rb

@@ -0,0 +1,40 @@
+require "test_helper"
+
+class Session::DevicesTest < ActiveSupport::TestCase
+  setup do
+    @session = sessions(:david)
+    @identity = @session.identity
+  end
+
+  test "destroying session destroys associated devices" do
+    device = ApplicationPushDevice.register(
+      session: @session,
+      token: "test_token",
+      platform: "apple",
+      name: "Test iPhone"
+    )
+
+    assert_difference -> { ApplicationPushDevice.count }, -1 do
+      @session.destroy
+    end
+
+    assert_nil ApplicationPushDevice.find_by(id: device.id)
+  end
+
+  test "destroying session does not destroy devices from other sessions" do
+    other_session = sessions(:kevin)
+
+    device = ApplicationPushDevice.register(
+      session: other_session,
+      token: "other_token",
+      platform: "apple",
+      name: "Other iPhone"
+    )
+
+    assert_no_difference -> { ApplicationPushDevice.count } do
+      @session.destroy
+    end
+
+    assert ApplicationPushDevice.exists?(device.id)
+  end
+end