fix(ci): satisfy clippy 1.97 so the Check job goes green

[MotherSphere]

The Check job has been red on all three runners since the toolchain
moved to 1.97: cargo clippy -- -D warnings reports 4 errors in the
bin target.

• app.rs / stats.rs: three descending sort_by comparators become
  sort_by_key(|b| Reverse(..)) (clippy::unnecessary_sort_by). Both
  APIs are stable sorts and the key is the same value the comparator
  read, so the resulting order - ties included - is unchanged.
• handlers/import.rs: drop the redundant & on an eprintln! argument
  (clippy::useless_borrows_in_formatting).
• steam/mod.rs: move mod tests below full_import
  (clippy::items_after_test_module). Pure relocation, no line edited.

Also clears the lints that only --all-targets surfaces, so
cargo clippy --workspace --all-targets -- -D warnings is clean too:
crate-level //! header in the integration tests, struct-update syntax
instead of a post-default field assignment, .filter(is_game), and a
documented allow(dead_code) on the mock payload structs whose unread
fields exist to pin the API response shape.