This is a definite work in progress. In particular,
* there's no src/web.rs support yet so it can't be used,
* the code is surprisingly complex, and there's almost no tests so far.
I want to at least get complete branch coverage.
* I may still go back to time_sec rather than time_90k to save RAM and
flash.
I simplified the approach a bit from the earlier goal in design/api.md.
In particular, there's no longer the separate concept of "observation"
vs "prediction". Now the predictions are just observations that extend a
bit beyond now. They may be flushed prematurely and I'll try living with
that to avoid making things even more complex.
This is mostly untested and useless by itself, but it's a starting
point. In particular:
* there's no way to set up signals or add/remove/update events yet
except by manual changes to the database.
* if you associate a signal with a camera then remove the camera,
hitting /api/ will error out.
