Add a missing minimum value on Element.rotation_angle

While working on #883 (closed), I noticed that Element.rotation_angle is shown in the OpenAPI documentation to accept values between -32768 and 359. This does not reflect what the database currently accepts, as there is a check constraint on [0, 359]. It turns out a MinValueValidator is missing so Spectacular cannot detect that minimum value, so this adds it, which causes a useless migration.

This does not affect the database in any way, and the data was already properly checked, this is only a matter of documentation.