test(auth): deflake login rate-limit burst test under CPU load (#10)

backend/tests/api_auth.rs

@@ -659,7 +659,8 @@ async fn login_no_user_branch_runs_argon2_for_timing_equalisation(pool: PgPool)
 async fn login_rate_limited_under_burst_pressure(pool: PgPool) {
     let h = common::harness_with_auth_rate_limit(pool, 1, 3);
 
-    // Register a victim so the wrong-password branch is real work.
+    // One register hit first — register and login share the one bucket,
+    // so this exercises the cross-endpoint limit (and consumes a token).
     let _ = h
         .app
         .clone()
@@ -667,9 +668,18 @@ async fn login_rate_limited_under_burst_pressure(pool: PgPool) {
         .await
         .unwrap();
 
-    // Register consumed one token from the burst-3 bucket. Fire 30
-    // wrong-password logins back-to-back; with per_sec=1 the refill
-    // is too slow to keep up and at least one must come back 429.
+    // Fire 30 logins back-to-back; at least one must come back 429.
+    //
+    // We send an EMPTY password on purpose. `login` calls the rate
+    // limiter FIRST, then short-circuits empty credentials with a 400
+    // BEFORE any argon2 hash or DB lookup — so each attempt still
+    // consumes a token but is near-instant. That makes the burst drain
+    // the bucket far faster than the per_sec=1 refill regardless of how
+    // loaded the CI box is. (A real wrong-password attempt runs argon2,
+    // ~1s under load, which lets the 1/sec refill keep exact pace with
+    // the loop and the bucket never empties — that was the old flake.
+    // The bucket math itself is covered by rate_limit.rs's unit test;
+    // here we only need to prove the limiter is wired into the route.)
     let mut saw_429 = false;
     for _ in 0..30 {
         let resp = h
@@ -677,7 +687,7 @@ async fn login_rate_limited_under_burst_pressure(pool: PgPool) {
             .clone()
             .oneshot(common::post_json(
                 "/api/v1/auth/login",
-                json!({ "username": "victim", "password": "wrong" }),
+                json!({ "username": "victim", "password": "" }),
             ))
             .await
             .unwrap();