fix: honest costs in streams, oracle-sized timeouts, history & log cleanup

Cost showed $0 for every streamed request: the X-Cortiq-Cost-USD header is
written before generation, when the cost cannot be known. The gateway now
ships the final number in a closing SSE event (`cortiq_cost_usd`, alongside
the token estimate when the provider sent no usage), and the console prefers
it over the header. Money renders with 4 decimals, the last two in a smaller
font — $0.0006 stays readable instead of collapsing to $0.

Timeouts, both ends: the router default was 800 ms, silently degrading every
prompt the hosted router escalates to its oracle (~10 s) — now 15 s. Provider
clients had NO timeout at all (a dead OpenRouter host hung requests forever) —
now a 10 s connect timeout, with no overall cap so long generations and
streams are never cut.

Benchmark history: per-run × and a Clear-all button. Logs view: a clear
button backed by DELETE /admin/api/logs.

src/admin/mod.rs

@@ -185,7 +185,7 @@ pub fn api_routes(admin_token: String) -> Router<SharedState> {
         .route("/admin/api/requests", get(get_requests))
         .route("/admin/api/test", post(run_test))
         .route("/admin/api/test/stream", post(run_test_stream))
-        .route("/admin/api/logs", get(get_logs))
+        .route("/admin/api/logs", get(get_logs).delete(clear_logs))
         .route("/admin/api/system", get(get_system))
         .route("/admin/api/restart", post(restart_gateway))
         .route("/admin/api/bench", get(bench_list).post(bench_start))
@@ -2113,6 +2113,11 @@ async fn get_logs(Query(q): Query<RecentQuery>) -> ApiResult {
     ok(json!({ "lines": crate::logbuf::snapshot(limit) }))
 }
 
+async fn clear_logs() -> ApiResult {
+    crate::logbuf::clear();
+    ok(json!({ "ok": true }))
+}
+
 /// Host metrics snapshot: CPU, RAM, free disk at the models dir, gateway RSS.
 async fn get_system(State(state): State<SharedState>) -> ApiResult {
     let snap = state.sysmon.snapshot();