When did a Celestial enter Tiz?
The celestial entered Tiz when you started a new game, then left him as the credits were ending.

You are the Celestial occupying Tiz's body.

The Celestial realm that Ouroboros is trying to break into is the real world (the 3DS camera even turns on and blends your face into the background during the final battle after Ouroboros says that he's approaching the Celestial Realm). Ouroboros is also talking directly to you when he asks if pretending to care about the party has alleviated your boredom.

The AR movie at the start is supposed to be Anges briefly breaking into the real world (the Celestial Realm) to beg you to intervene.

The extra movie is a sneak peak of Bravely Second. He wakes up from the coma because the celestial (you) came back to him for the next game.