Fails to build with postgres feature

## Summary

I'm interested in using postgres for my database, but `cargo build --features postgres` fails with a type error. The `solo-machine-core` crate does support this as a feature, but isn't working as expected.

## Replicating the error

1. Clone repo
2. `cargo build --features postgres`
3. Observe the following error:

```
...
error[E0271]: type mismatch resolving ` as Executor<'_>>::Database == Sqlite`
--> solo-machine-core/src/model/chain/chain_keys.rs:94:16
|
94 | .fetch_all(executor)
| --------- ^^^^^^^^ expected `Sqlite`, found `Postgres`
| |
| required by a bound introduced by this call
|
```

Run `cargo build --features postgres` to reproduce the failure, then inspect `solo-machine-core/src/model/chain/chain_keys.rs` around line 94 and the related database feature configuration. Done means the postgres-feature build completes without the reported `Sqlite` versus `Postgres` type error.

Written by the indexing model from the issue text.