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

models/user_preference.go

@@ -0,0 +1,54 @@
+package models
+
+import (
+	"context"
+	"errors"
+	"time"
+
+	"gorm.io/gorm"
+	"gorm.io/gorm/clause"
+)
+
+// UserPreference is one user's widget preference
+// (golem15_feedback_user_preferences, one row per user who ever changed
+// it). The hidden column is the signal, not the row: a row may hold
+// hidden=false after the user showed the widget again.
+type UserPreference struct {
+	ID        uint      `gorm:"column:id;primaryKey"`
+	UserID    uint      `gorm:"column:user_id"`
+	Hidden    bool      `gorm:"column:hidden"`
+	CreatedAt time.Time `gorm:"column:created_at"`
+	UpdatedAt time.Time `gorm:"column:updated_at"`
+}
+
+// TableName is golem15_feedback_user_preferences.
+func (UserPreference) TableName() string { return "golem15_feedback_user_preferences" }
+
+// IsWidgetHidden is UserPreference::isWidgetHidden: the user's hidden flag,
+// false without a row.
+func IsWidgetHidden(ctx context.Context, db *gorm.DB, userID uint) (bool, error) {
+	var p UserPreference
+	err := db.WithContext(ctx).Select("hidden").Where("user_id = ?", userID).Take(&p).Error
+	if errors.Is(err, gorm.ErrRecordNotFound) {
+		return false, nil
+	}
+	if err != nil {
+		return false, err
+	}
+	return p.Hidden, nil
+}
+
+// SetWidgetHidden is UserPreference::setWidgetHidden (updateOrCreate on
+// user_id): one atomic upsert of the user's row.
+func SetWidgetHidden(ctx context.Context, db *gorm.DB, userID uint, hidden bool) error {
+	now := time.Now()
+	row := UserPreference{UserID: userID, Hidden: hidden, CreatedAt: now, UpdatedAt: now}
+	return db.WithContext(ctx).Clauses(clause.OnConflict{
+		Columns:   []clause.Column{{Name: "user_id"}},
+		DoUpdates: clause.AssignmentColumns([]string{"hidden", "updated_at"}),
+	}).Create(&row).Error
+}
+
+func init() {
+	Register(UserPreference{})
+}