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/games.rs

@@ -5,7 +5,7 @@ use common::{
     environment::{TestEnvironment, with_test_environment},
 };
 
-mod common;
+pub mod common;
 
 #[actix_rt::test]
 async fn get_games() {