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/src/database/models/friend_item.rs

@@ -105,7 +105,7 @@ impl DBFriend {
             created: row.created,
             accepted: row.accepted,
         })
-        .filter(|x| accepted.map(|y| y == x.accepted).unwrap_or(true))
+        .filter(|x| accepted.is_none_or(|y| y == x.accepted))
         .collect::<Vec<_>>();
 
         Ok(friends)