Add postgres storage adapter for samod

[jmoggr]

I shipped the Samod integration without it ever touching our DB?! Yup, this is equal parts embarrassing and amusing while also being a surprisingly small issue.

Thanks to the decentralized nature of Automerge we can largely get by without persistence on the server. Documents are synchronized with the server on connection from the browser local storage. So unless the user changes browsers, computers, or clears their browser cache, nothing would ever appear wrong.

Besides missing InMemoryStorage, how did I miss this? Because I often have both Firefox and Chrome open when testing/debugging/developing, so when I cleared the browser cache from one with the server shutdown, the open documents were effectively restored from the other browsers cache as soon as the server started again. The problems really started when trying to use the staging database locally.

The storage adapter tests were re-written from the automerge-repo typescript, I'm going to try getting them upstreamed. I opened an upstream PR here

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backend startup has changed. Samod tries to read from the database on initialization and will error if the migrations have not run. Now backend blocks until the migrations are complete instead of using a status gate.

The status gate gave us the ability to run long migrations without worrying about the systemd service timing out on startup. However continuing to support it would increase complexity by a non-trivial amount. Removing it simplifies things and will automatically fail the deployment if the migrations fail.

[epatters]

Haha, I guess our app being mostly local-first means that it mostly works without a persistent storage ;)

This looks great. And thanks all for trying to upstream the storage adapter tests.