When did the Catholic Church decide 'daily bread" is its Eucharist?

The 2nd paragraph of CCC 2837 you quoted from the 1997 Catechism has a footnote (#131) pointing to St. Augustine's Sermon 57, which is a sermon on the Lord's prayer. Probably this citation is preferred to the one used in the 1566 Catechism of the Council of Trent because although St. Ambrose was earlier than St. Augustine, the citation is probably not from him (see Geremia's answer).

Paragraph 7 of Sermon 57 is on the petition "Give us this day our daily bread." Here's the complete paragraph (emphasis mine):

There remain now the petitions for this life of our pilgrimage; therefore follows, “Give us this day our daily bread.”⁹[Mt 6:11] Give us eternal things, give us things temporal. Thou hast promised a kingdom, deny us not the means of subsistence. Thou wilt give everlasting glory with Thyself hereafter, give us in this earth temporal support. Therefore is it “day by day,” and “to-day,” that is, in this present time. For when this life shall have passed away, shall we ask for daily bread then? For then it will not be called, “day by day,” but “to-day.” Now it is called, “day by day,” when one day passes away, and another day succeeds. Will it be called “day by day,” when there will be one eternal day? This petition for daily bread is doubtless to be understood in two ways, both for the necessary supply of our bodily food, and for the necessities of our spiritual support. There is a necessary supply of bodily food, for the preservation of our daily life, without which we cannot live. This is food and clothing, but the whole is understood in a part. When we ask for bread, we thereby understand all things. There is a spiritual¹⁰ food also which the faithful know, which ye too will know, when ye shall receive it at the altar of God. This also is “daily Bread,” necessary only for this life. For shall we receive the Eucharist when we shall have come to Christ Himself, and begun to reign with Him for ever? So then the Eucharist is our daily bread; but let us in such wise receive it, that we be not refreshed in our bodies only, but in our souls. For the virtue which is apprehended there, is unity, that gathered together into His body, and made His members, we may be what we receive. Then will it be indeed our daily bread. Again, what I am handling before you now is “daily bread;” and the daily lessons which ye hear in church, are daily bread, and the hymns ye hear and repeat are daily bread. For all these are necessary in our state of pilgrimage. But when we shall have got to heaven, shall we hear the word,¹¹ we who shall see the Word Himself, and hear the Word Himself, and eat and drink Him as the angels do now? Do the angels need books, and interpreters, and readers? Surely not. They read in seeing, for the Truth Itself they see, and are abundantly satisfied from that fountain, from which we obtain some few¹² drops. Therefore has it been said touching our daily bread, that this petition is necessary for us in this life.

While this answer cannot give the exact WHEN that the Catholic Church decided that the "daily bread" is its Eucharist, the article Fathers of the Church on the Eucharist by Fr. Burns K. Seeley, SSJC, PhD provided a quote from an earlier Church Father, St. Cyprian of Carthage (c. 200-258 AD).

Quote from paragraph 18 of St. Cyprian's Treatise 4 - On the Lord's Prayer, c. 252 AD (emphasis mine):

As the prayer goes forward, we ask and say, "Give us this day our daily bread". And this may be understood both spiritually and literally, because either way of understanding it is rich in divine usefulness to our salvation. For Christ is the bread of life; and this bread does not belong to all men, but it is ours. And according as we say, "Our Father", because He is the Father of those who understand and believe; so also we call it "our bread", because Christ is the bread of those who are in union with His body. And we ask that this bread should be given to us daily, that we who are in Christ, and daily receive the Eucharist for the food of salvation, may not, by the interposition of some heinous sin, by being prevented, as withheld and not communicating, from partaking of the heavenly bread, be separated from Christ's body, as He Himself predicts, and warns, "I am the bread of life which came down from heaven. If any man eat of my bread, he shall live for ever: and the bread which I will give is my flesh, for the life of the world". John 6:58 When, therefore, He says, that whoever shall eat of His bread shall live for ever; as it is manifest that those who partake of His body and receive the Eucharist by the right of communion are living, so, on the other hand, we must fear and pray lest any one who, being withheld from communion, is separate from Christ's body should remain at a distance from salvation; as He Himself threatens, and says, "Unless you eat the flesh of the Son of man, and drink His blood, you shall have no life in you." John 6:53 And therefore we ask that our bread — that is, Christ — may be given to us daily, that we who abide and live in Christ may not depart from His sanctification and body.

To show that St. Cyprian meant Eucharist as The Real Presence, read other quotations from St. Cyprian in the article The Early Christians Believed in the Real Presence.