db/diesel: Better docs / accessibility for schema changes and code gen

[drahnr]

Previously we had a doc comment:

/// [diesel](https://diesel.rs/) generated schema
///     
/// ```sh     
/// cargo binstall diesel_cli     
/// sqlite3 -init ./src/db/migrations/001-init.sql ephemeral_setup.db ""     
/// diesel setup --database-url=./ephemeral_setup.db     
/// diesel print-schema > src/db/schema.rs```

but really we should have either a makefile target or similar to automate it away. That is, if we can meaningfully represent everything in the schema.