Fix mobile login "Record not found" for unseeded instances (#916)

* Auto-create mobile OAuth application when missing (#912)

Self-hosted users who set up their instance without running `db:seed`
(or reset their database) got "Record not found" on mobile login because
`MobileDevice.shared_oauth_application` used `find_by!` which raises
when the "Sure Mobile" Doorkeeper application does not exist.

Switch to `find_or_create_by!` so the record is created transparently
on first use, matching the attributes from the seed file.

* Nice Claude Code suggestion

---------

Co-authored-by: Juan José Mata <jjmata@jjmata.com>

test/models/mobile_device_test.rb

@@ -1,7 +1,23 @@
 require "test_helper"
 
 class MobileDeviceTest < ActiveSupport::TestCase
-  # test "the truth" do
-  #   assert true
-  # end
+  setup do
+    MobileDevice.instance_variable_set(:@shared_oauth_application, nil)
+  end
+
+  teardown do
+    MobileDevice.instance_variable_set(:@shared_oauth_application, nil)
+  end
+
+  test "shared_oauth_application auto-creates application when missing" do
+    Doorkeeper::Application.where(name: "Sure Mobile").destroy_all
+
+    assert_difference("Doorkeeper::Application.count", 1) do
+      app = MobileDevice.shared_oauth_application
+      assert_equal "Sure Mobile", app.name
+      assert_equal MobileDevice::CALLBACK_URL, app.redirect_uri
+      assert_equal "read_write", app.scopes.to_s
+      assert_not app.confidential
+    end
+  end
 end