Reapply "feat(loyalty): EnakPoint & EnakCoin" (#32)

This reverts commit 4e24f9bbb0.

internal/entities/loyalty_setting_change.go

@@ -0,0 +1,34 @@
+package entities
+
+import (
+	"time"
+
+	"github.com/google/uuid"
+	"gorm.io/gorm"
+)
+
+// LoyaltySettingChange records one change to a loyalty setting: which key, from what,
+// to what, by whom (docs/prd-point-coin.md F2). Append-only.
+type LoyaltySettingChange struct {
+	ID             uuid.UUID `gorm:"type:uuid;primary_key;default:gen_random_uuid()" json:"id"`
+	OrganizationID uuid.UUID `gorm:"type:uuid;not null" json:"organization_id"`
+	// Nil for an organization setting.
+	OutletID *uuid.UUID `gorm:"type:uuid" json:"outlet_id"`
+	Key      string     `gorm:"not null;size:100" json:"key"`
+	// Nil when the key had no stored value, or has none any more.
+	OldValue  *string   `gorm:"type:text" json:"old_value"`
+	NewValue  *string   `gorm:"type:text" json:"new_value"`
+	ChangedBy uuid.UUID `gorm:"type:uuid;not null" json:"changed_by"`
+	CreatedAt time.Time `gorm:"autoCreateTime" json:"created_at"`
+}
+
+func (c *LoyaltySettingChange) BeforeCreate(tx *gorm.DB) error {
+	if c.ID == uuid.Nil {
+		c.ID = uuid.New()
+	}
+	return nil
+}
+
+func (LoyaltySettingChange) TableName() string {
+	return "loyalty_setting_changes"
+}