style(backend): rustfmt the whole tree; gate cargo fmt --check in CI

The backend had never been run through rustfmt. Doing it in one mechanical pass (134 files)
so no future functional diff is buried under formatting churn, then gating `cargo fmt
--check` in checks.yml so it stays clean.

Formatting only — no logic, SQL, or behaviour changed. Verified after the reformat:
cargo test 56 passed, clippy --all-targets -D warnings clean, cargo fmt --check clean.

This is the deferred cleanup noted when CI's Format step was first left out.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/src/services/maintenance.rs

@@ -87,11 +87,7 @@ pub async fn startup_recovery(pool: &PgPool) {
 /// - drops expired SSE tickets (30s TTL but the map keeps the slot until pruned)
 ///
 /// Cadence is 1h — fine for both jobs at our scale.
-pub fn spawn_periodic_tasks(
-    pool: PgPool,
-    rate_limiter: RateLimiter,
-    sse_tickets: SseTicketStore,
-) {
+pub fn spawn_periodic_tasks(pool: PgPool, rate_limiter: RateLimiter, sse_tickets: SseTicketStore) {
     tokio::spawn(async move {
         let mut tick = tokio::time::interval(Duration::from_secs(3600));
         // Fire the first tick immediately, then hourly.