If I move onto a battlefield as an attacker, can I then play Janna?

Yes, you can play Janna, Savior in this situation, provided you follow the placement restriction for units.

According to the RiftJudge FAQ #10964, you can play Janna, Savior in a showdown while you are the attacker, as long as you play her to a battlefield you control.

Important Considerations:

• Location Requirement: Units must be played to a battlefield you control (or your base). When you move into an occupied battlefield to initiate combat, you do not control that battlefield (it is contested). Therefore, you cannot play Janna, Savior to the contested battlefield itself.
• Legal Placement: You may play Janna, Savior to a different battlefield that you already control, or to your base.
• Timing: Because Janna, Savior has the [Reaction] keyword, she can be played at any time, including during the Closed state of a showdown.

Summary: You can play her during your combat showdown as a reaction, but you must place her at a location you control (your base or a different friendly battlefield), not the battlefield currently under combat.