style(server): cargo fmt for SPEC-004 Task 2 + Task 4

Pure rustfmt reflow of the Task 2 (machine_uid dedup) and Task 4 (session
reaping) code; no logic change. The CI Build-Server-Linux job gates on
cargo fmt --check, which the two feature commits failed because local
validation ran check/clippy/test but not fmt --check. fmt --check, check,
and clippy -D warnings all clean now.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

server/src/db/machines.rs

@@ -399,7 +399,10 @@ mod tests {
         let m1 = upsert_machine(&pool, agent, "LEGACY-HOST", true, None)
             .await
             .expect("legacy upsert (no uid)");
-        assert_eq!(m1.machine_uid, None, "legacy row must have NULL machine_uid");
+        assert_eq!(
+            m1.machine_uid, None,
+            "legacy row must have NULL machine_uid"
+        );
 
         let m2 = upsert_machine(&pool, agent, "LEGACY-HOST-RENAMED", true, None)
             .await