fix(manager-core): F-P-002 move Argon2id verify off the Tokio async worker

verify_password (Argon2id, OWASP defaults m=19456 KiB, t=2) is CPU-bound
at tens-to-hundreds of ms and was invoked synchronously on the Tokio
worker. Worse, verify_api_key Argon2-verifies every candidate sharing
the 8-char prefix — a hot user with N keys serialized every admin
request behind N×Argon2.

Wrap each call site in tokio::task::spawn_blocking:
- auth_middleware::verify_api_key (per request carrying a Bearer key)
- auth_api::login (admin login)
- users_service::login (data-plane app-user login, both real-hash and
  TIMING_FLAT_DUMMY_HASH branches)

Cold-cache login is now ~2× current latency due to one spawn_blocking
hop, but the worker no longer parks on Argon2 so steady-state under
load is dramatically better. The LRU cache for the hot-path (token →
principal) is finding F-P-009 — separate commit.

AUDIT.md anchor: F-P-002.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/manager-core/src/auth_api.rs

@@ -95,7 +95,17 @@ async fn login(State(state): State<AuthState>, Json(input): Json<LoginRequest>)
         None => (crate::auth::TIMING_FLAT_DUMMY_HASH.to_string(), None, false),
     };
 
-    let password_ok = verify_password(&stored_hash, &input.password);
+    // F-P-002: Argon2id verify off the async worker.
+    let password = input.password.clone();
+    let password_ok = match tokio::task::spawn_blocking(move || verify_password(&stored_hash, &password))
+        .await
+    {
+        Ok(b) => b,
+        Err(err) => {
+            tracing::error!(?err, "verify_password spawn_blocking join failed");
+            return internal_error();
+        }
+    };
     if !password_ok || user_id.is_none() || !is_active {
         return invalid_credentials();
     }