chore(backend): satisfy cargo fmt

`checks.yml` runs `cargo fmt --check`, and it has been failing since the round-1
audit fixes: I gated those on `cargo build` and `cargo clippy` but never ran fmt,
so three files drifted then and eight more this round. Pure formatting — no
behaviour change; clippy stays at zero and all 70 backend tests still pass.

Worth noting for next time: clippy passing is not evidence fmt does.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

backend/tests/failed_original_sweep.rs

@@ -86,7 +86,15 @@ async fn sweeps_only_long_failed_soft_deleted_uploads(pool: PgPool) {
 
     // Everything below is a near-miss that must survive.
     // A healthy live upload — the catastrophic case if the predicate were ever loosened.
-    let live = seed_upload(&pool, event_id, user_id, "done", None, "originals/e/live.jpg").await;
+    let live = seed_upload(
+        &pool,
+        event_id,
+        user_id,
+        "done",
+        None,
+        "originals/e/live.jpg",
+    )
+    .await;
     // Failed but still inside the retention window: the recovery window is the entire point
     // of keeping the file, so reclaiming it early would defeat the fix it protects.
     let recent = seed_upload(