feat: Sprint 3 — benchmark + conversion funnel + invoice PDF

- 3.1: Add criterion benchmark for zclaw-growth TF-IDF retrieval
  (indexing throughput, query scoring latency, top-K retrieval)
- 3.2: Extend admin-v2 Usage page with recharts funnel chart
  (registration → trial → paid conversion) and daily trend bar chart
- 3.3: Add invoice PDF export via genpdf (Arial font, Windows)
  with GET /api/v1/billing/invoices/{id}/pdf handler

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

crates/zclaw-saas/src/billing/mod.rs

@@ -4,6 +4,7 @@ pub mod types;
 pub mod service;
 pub mod handlers;
 pub mod payment;
+pub mod invoice_pdf;
 
 use axum::routing::{get, post};
 
@@ -17,6 +18,7 @@ pub fn routes() -> axum::Router<crate::state::AppState> {
         .route("/api/v1/billing/usage/increment", post(handlers::increment_usage_dimension))
         .route("/api/v1/billing/payments", post(handlers::create_payment))
         .route("/api/v1/billing/payments/{id}", get(handlers::get_payment_status))
+        .route("/api/v1/billing/invoices/{id}/pdf", get(handlers::get_invoice_pdf))
 }
 
 /// 支付回调路由（无需 auth — 支付宝/微信服务器回调）