chore(clippy): enable and fix many stricter lints (#3783)

* chore(clippy): enable and fix many stricter lints

These ensure that the codebase uses more idiomatic, performant, and
concise language constructions.

* chore: make non-Clippy compiler warnings also deny by default

apps/labrinth/tests/common/database.rs

@@ -1,5 +1,3 @@
-#![allow(dead_code)]
-
 use labrinth::{database::redis::RedisPool, search};
 use sqlx::{PgPool, postgres::PgPoolOptions};
 use std::time::Duration;