Where do Catholics believe the historical narrative of the Bible begins?

The Catholic Church teaches, in the decisions of the 1909 Pontifical Biblical Commission (Latin original), that at least

The first three Chapters of Genesis contain narratives that correspond to objectively real and historically true events (rerum vere gestarum narrationes quae scilicet obiectivae realitati et historicae veritati respondeant), no myths, no mere allegories or symbols of religious truths, no legends.

Thus, there is no way that the Deluge is "prehistorical", if the first 3 chapters of Genesis are "historically true events."

The Catechism of the Catholic Church says (#105), "God is the author of Sacred Scripture." Further, (#106)

[The human authors of scripture]consigned to writing whatever he wanted written and no more."

Therefore, there is no part of the scriptures that is merely human and potentially erroneous. God is Truth, and can neither deceive nor be deceived. Thus, I believe that the answer to your question is that all of Genesis is to be considered historical, unless there is a reason to believe that the literal sense is different (as has been suggested with Genesis 1).

If you look at Pius XII's Divino Afflante Spiritu, you will find this spelled out:

having been written under the inspiration of the Holy Spirit, they have God for their author, and as such were handed down to the Church herself."[3] When, subsequently, some Catholic writers, in spite of this solemn definition of Catholic doctrine, by which such divine authority is claimed for the "entire books with all their parts" as to secure freedom from any error whatsoever, ventured to restrict the truth of Sacred Scripture solely to matters of faith and morals, and to regard other matters, whether in the domain of physical science or history, as "obiter dicta" and - as they contended - in no wise connected with faith, Our Predecessor of immortal memory, Leo XIII in the Encyclical Letter Providentissimus Deus, published on November 18 in the year 1893, justly and rightly condemned these errors and safe-guarded the studies of the Divine Books by most wise precepts and rules.

Further,

The first and greatest care of Leo XIII was to set forth the teaching on the truth of the Sacred Books and to defend it from attack. Hence with grave words did he proclaim that there is no error whatsoever if the sacred writer, speaking of things of the physical order "went by what sensibly appeared" as the Angelic Doctor says,[5] speaking either "in figurative language, or in terms which were commonly used at the time, and which in many instances are in daily use at this day, even among the most eminent men of science." For "the sacred writers, or to speak more accurately - the words are St. Augustine's - [6] the Holy Spirit, Who spoke by them, did not intend to teach men these things - that is the essential nature of the things of the universe - things in no way profitable to salvation"; which principle "will apply to cognate sciences, and especially to history,"[7]

And also

Finally it is absolutely wrong and forbidden "either to narrow inspiration to certain passages of Holy Scripture, or to admit that the sacred writer has erred," since divine inspiration "not only is essentially incompatible with error but excludes and rejects it as absolutely and necessarily as it is impossible that God Himself, the supreme Truth, can utter that which is not true. This is the ancient and constant faith of the Church."[9]

From these considerations, I would say that "for the sake of salvation" from Dei Verbum in this quote should be interpreted as the motivation for God's revelation in the scriptures and not as a restriction of inerrancy to moral and theological subjects.

Therefore, since everything asserted by the inspired authors or sacred writers must be held to be asserted by the Holy Spirit, it follows that the books of Scripture must be acknowledged as teaching solidly, faithfully and without error that truth which God wanted put into sacred writings (5) for the sake of salvation.

In the area historical accuracy, the Old Testament has been proven a very reliable witness to the ancient, ancient world. One should be careful about finding historical inaccuracies there, because the authors of the Old Testament, even at the latest date argued for the time of writing, were far closer to the events and society of the times they depict than we are.

A little anachronism is to be expected in human language. For instance, we often talk about things happening in Israel or Turkey when it is only a geographical accident that they are included in the current borders of the political entities that bear those names today. That doesn't make the statements any less true.

The problem with that question is that it makes a very difficult assumption: that the Church has an official position as to when the historical begins. Frankly, it doesn't.

The Church's view of Genesis can be best summed up by, "it means what it means." Yes, that is tautology, but it is necessarily so — we know that the Bible is 100% accurate in faith and morals as intended by the author, but it is very difficult to narrow down the theology beyond that. Further, it is not impossible for the author to make an error in matters historical as history is not strictly limited by faith and morals. (Placement of the Medes in the book of Daniel, for example, is generally considered an anachronism, but it does make sense if taken more symbollically).

On a personal note: I view the histories as starting with the book of Joshua and continuing from there. For, while there is definitely som mytho-symbolic content in Judges, my reading is that, overall, the concern of the documents is the historical progression of the nation. Yes, that does omit the entire pentateuch, but, while I will not deny that there is most certainly history in those documents, picking apart history from legend in those five is a bit tricky.