Is the intention to combine prayers (Jama') obligatory before beginning them, and is the prayer invalidated by not hearing the Imam command the intention to combine, and what if I recall the intention to combine in my heart?

If you combined the prayers without the intention of combining them when you started the first one, it is hoped that this will be sufficient for you. You may adopt the view of the scholars who did not stipulate this intention, and it is a strong opinion. The Prophet, peace and blessings be upon him, did not command his Companions to have the intention for combining or shortening prayers. Therefore, your prayer is valid, and you do not need to repeat it.