refactor: Implement From<sqlx::Error> and clean up fix_error (#3484)

In the database crates for atuin-server, there is `fn fix_error`. This
PR implements `From<sqlx::Error>` on `DbError` which makes it possible
to mostly use `?` to bubble up the errors.

There are cases where `?` is not being used e.g.
```rust
    async fn get_session(&self, token: &str) -> DbResult<Session> {
        sqlx::query_as("select id, user_id, token from sessions where token = $1")
            .bind(token)
            .fetch_one(&self.pool)
            .await
            .map_err(fix_error)
            .map(|DbSession(session)| session)
    }
```

There are two options
## 1. Use `Into::into`
```rust
    async fn get_session(&self, token: &str) -> DbResult<Session> {
        sqlx::query_as("select id, user_id, token from sessions where token = $1")
            .bind(token)
            .fetch_one(&self.pool)
            .await
            .map_err(fix_error)
            .map(|DbSession(session)| session)
    }
```

## 2. Create a variable and use `?`
```rust
    async fn get_session(&self, token: &str) -> DbResult<Session> {
        let session = sqlx::query_as("select id, user_id, token from sessions where token = $1")
            .bind(token)
            .fetch_one(&self.pool)
            .await
            .map(|DbSession(session)| session)?;
        Ok(session)
    }
```

I chose to do option 1 as it was just a find/replace but say the word
and I'll convert them all to option 2

## Checks
- [X] I am happy for maintainers to push small adjustments to this PR,
to speed up the review cycle
- [X] I have checked that there are no existing pull requests for the
same thing

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