feat: golem15.feedback plugin with the widget config route

- plugin golem15.feedback (requires golem15.user), its config, lang (en, pl), models and migrations
- typed settings singleton golem15_feedback_settings with the WinterCMS defaults and a widget key generated on first creation
- GET /_feedback/api/v1/{key}/config behind the constant-time key check and the fail-closed Origin allow-list, with the feedback-config and feedback-submit buckets

config/config.yaml

@@ -0,0 +1,8 @@
+g15_office:
+  # The G15Office (support portal) connection every submission is filed
+  # through. Nothing is sent while any of the three is empty. Override them
+  # with SUMMER_GOLEM15__FEEDBACK__G15_OFFICE__BASE_URL, __TOKEN and
+  # __PROJECT; never commit the token.
+  base_url: ""
+  token: ""
+  project: ""