Footage: "try again now" books a step now, not when the clip was due

[pallaoro]

A read kept any booked step that wasn't overdue, even one booked hours out for clips waiting on Google Drive. With a Drive connection only a delivery starts an import (reads start none), so "Try Google Drive again now" waited for that old booking: on a live project the next step stayed booked for an hour after the retry.

Now a read keeps the booked step only when it is due no later than the step the read would book (a minute out, or the time the read passes when nothing can move before then). Otherwise it books the earlier one. The later booking still fires and finds nothing more to do.

• No extra bookings while nothing can move: those reads pass the clips' due time, and the booking already matches it.
• Test: scenario 7b now asserts the retry books a step a minute out. It fails on the old code ("a step is booked for now, not for when the clip was due") and passes with the change.
• Typecheck clean, 185 unit tests, end to end ALL OK.