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

controllers/api/winter_errors.go

@@ -0,0 +1,99 @@
+package api
+
+import (
+	"bytes"
+	"encoding/json"
+	"net/http"
+	"sort"
+
+	"git.golem15.com/golem15/summercms/modules/wire"
+)
+
+// writeJSON answers like Laravel's response()->json behind WinterCMS's
+// json.response middleware: the JSON body with Cache-Control
+// "no-cache, private". The plugin keeps its own writers because a plugin in
+// its own repository cannot import an application's controllers.
+func writeJSON(w http.ResponseWriter, status int, v any) {
+	w.Header().Set("Cache-Control", "no-cache, private")
+	wire.WriteJSON(w, status, v)
+}
+
+// writeError is the widget routes' {"error":true,"message":...} envelope.
+func writeError(w http.ResponseWriter, status int, message string) {
+	writeJSON(w, status, errorBody{Error: true, Message: message})
+}
+
+type errorBody struct {
+	Error   bool   `json:"error"`
+	Message string `json:"message"`
+}
+
+// writeOpaque500 is the house opaque 500 for a failure that is not the
+// client's (no database, a failed write).
+func writeOpaque500(w http.ResponseWriter) {
+	wire.WriteOpaque500(w)
+}
+
+// orderedErrors marshals a Laravel message bag with its keys in rule order
+// (lagoon.ErrorKeys), the remaining keys sorted after them.
+type orderedErrors struct {
+	errs  map[string][]string
+	order []string
+}
+
+func (o orderedErrors) MarshalJSON() ([]byte, error) {
+	var buf bytes.Buffer
+	buf.WriteByte('{')
+	seen := map[string]bool{}
+	first := true
+	write := func(k string) error {
+		msgs, ok := o.errs[k]
+		if !ok || seen[k] {
+			return nil
+		}
+		seen[k] = true
+		if !first {
+			buf.WriteByte(',')
+		}
+		first = false
+		kb, err := marshalNoEscape(k)
+		if err != nil {
+			return err
+		}
+		vb, err := marshalNoEscape(msgs)
+		if err != nil {
+			return err
+		}
+		buf.Write(kb)
+		buf.WriteByte(':')
+		buf.Write(vb)
+		return nil
+	}
+	for _, k := range o.order {
+		if err := write(k); err != nil {
+			return nil, err
+		}
+	}
+	rest := make([]string, 0, len(o.errs))
+	for k := range o.errs {
+		rest = append(rest, k)
+	}
+	sort.Strings(rest)
+	for _, k := range rest {
+		if err := write(k); err != nil {
+			return nil, err
+		}
+	}
+	buf.WriteByte('}')
+	return buf.Bytes(), nil
+}
+
+func marshalNoEscape(v any) ([]byte, error) {
+	var buf bytes.Buffer
+	enc := json.NewEncoder(&buf)
+	enc.SetEscapeHTML(false)
+	if err := enc.Encode(v); err != nil {
+		return nil, err
+	}
+	return bytes.TrimRight(buf.Bytes(), "\n"), nil
+}