test: cover the trusted_proxy X-Forwarded-For rate-limit gate

Two integration tests pinning both sides of the gate: trusted_proxy=true gives
each forwarded IP its own bucket; trusted_proxy=false ignores spoofed XFF and
shares the global bucket. Closes audit gap M7. Test-only, no behavior change.

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

backend/tests/api_auth.rs

@@ -9,6 +9,73 @@ fn creds(username: &str) -> serde_json::Value {
     json!({ "username": username, "password": "hunter2hunter2" })
 }
 
+/// A login request carrying a chosen `X-Forwarded-For`. Empty password so the
+/// handler consumes a rate-limit token then short-circuits with 400 before any
+/// argon2/DB work — the burst drains near-instantly regardless of CI load.
+fn login_from(xff: &str) -> axum::http::Request<axum::body::Body> {
+    axum::http::Request::builder()
+        .method("POST")
+        .uri("/api/v1/auth/login")
+        .header(header::CONTENT_TYPE, "application/json")
+        .header("x-forwarded-for", xff)
+        .body(axum::body::Body::from(
+            json!({ "username": "victim", "password": "" }).to_string(),
+        ))
+        .unwrap()
+}
+
+// The per-IP rate limiter's soundness hinges on one gate: X-Forwarded-For is
+// honored ONLY when AUTH_TRUSTED_PROXY is set. These two tests pin both sides of
+// that gate at the request level (the pure parser is unit-tested separately).
+
+#[sqlx::test(migrations = "./migrations")]
+async fn trusted_proxy_gives_each_forwarded_ip_its_own_bucket(pool: PgPool) {
+    let h = common::harness_with_auth_rate_limit_proxy(pool, 1, 2, true);
+    // Drain IP A's bucket (per_sec=1, burst=2) until it 429s.
+    let mut a_saw_429 = false;
+    for _ in 0..8 {
+        let resp = h.app.clone().oneshot(login_from("203.0.113.10")).await.unwrap();
+        if resp.status() == StatusCode::TOO_MANY_REQUESTS {
+            a_saw_429 = true;
+            break;
+        }
+    }
+    assert!(a_saw_429, "IP A must be rate-limited after draining its own bucket");
+    // A different forwarded IP has an independent bucket — its first hit is not 429.
+    let resp_b = h.app.clone().oneshot(login_from("198.51.100.20")).await.unwrap();
+    assert_ne!(
+        resp_b.status(),
+        StatusCode::TOO_MANY_REQUESTS,
+        "a distinct X-Forwarded-For hop must get its own bucket, not A's drained one"
+    );
+}
+
+#[sqlx::test(migrations = "./migrations")]
+async fn untrusted_proxy_ignores_forwarded_ip_and_shares_one_bucket(pool: PgPool) {
+    let h = common::harness_with_auth_rate_limit_proxy(pool, 1, 2, false);
+    // Every request carries a DISTINCT (spoofed) X-Forwarded-For, but the backend
+    // doesn't trust it — so they all fall back to the single global bucket, which
+    // drains and starts returning 429. If XFF were wrongly honored here, each
+    // unique IP would get a fresh bucket and none of these would ever 429.
+    let mut saw_429 = false;
+    for i in 0..12 {
+        let resp = h
+            .app
+            .clone()
+            .oneshot(login_from(&format!("10.0.0.{i}")))
+            .await
+            .unwrap();
+        if resp.status() == StatusCode::TOO_MANY_REQUESTS {
+            saw_429 = true;
+            break;
+        }
+    }
+    assert!(
+        saw_429,
+        "with trusted_proxy off, spoofed X-Forwarded-For must NOT dodge the shared limiter"
+    );
+}
+
 #[sqlx::test(migrations = "./migrations")]
 async fn register_creates_user_and_sets_session_cookie(pool: PgPool) {
     let h = common::harness(pool);