Db, direct interface, errors, and migrations

Adds a migration for an initial schema. Adds sqlx-cli to the flake
so that I use those migrations easily.

adds an UNTESTED "direct" interface, which should make writing ingestors
easier, and be very easy for spoor lexicons. It will probably also be
easy for other lexicons if they can fit into the Activity -> Session ->
Content interface.

Adds errors because I needed errors.

Since this is a library, and as you can see there are no tests, that
means that everything is *untested* which is fine because I want a MVP
not a stable app. For now.

flake.nix

@@ -42,6 +42,7 @@
           packages = (with pkgs; [
             # The package provided by our custom overlay. Includes cargo, Clippy, cargo-fmt,
             # rustdoc, rustfmt, and other tools.
+            sqlx-cli
             rustToolchain
           ]) ++ pkgs.lib.optionals pkgs.stdenv.isDarwin (with pkgs; [ libiconv ]);
         };