A Takeout photo with no sidecar and no date in its name was filed under
the day of the import, and the app dated uploads by modification time.
A new `exiftime` module reads DateTimeOriginal (then DateTimeDigitized)
with its offset. Takeout uses it for undated photos it copies out for a
thumbnail, and the daemon uses it for app uploads before falling back to
the modification time (B244).
