Add event_id to wedding_invitation body_params so it appears in the form for dynamic button URL

backend/whatsapp_templates.py

@@ -169,6 +169,7 @@ TEMPLATES: Dict[str, Dict[str, Any]] = {
             "event_date",      # body {{5}}
             "event_time",      # body {{6}}
             "guest_link",      # body {{7}}
+            "event_id",        # button {{1}} - for dynamic URL (not in body text)
         ],
         "button_type": "URL",
         "button_text": "הצבע על הזמנה",