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/handlers.rs

@@ -473,3 +473,77 @@ mod crypto {
         provided.len() >= 16 && expected.len() >= 16 && provided == expected
     }
 }
+
+// === 发票 PDF ===
+
+/// GET /api/v1/billing/invoices/:id/pdf — 下载发票 PDF
+pub async fn get_invoice_pdf(
+    State(state): State<AppState>,
+    Extension(ctx): Extension<AuthContext>,
+    Path(invoice_id): Path<String>,
+) -> SaasResult<axum::response::Response> {
+    // 查询发票（需属于当前账户）
+    let invoice: Invoice = sqlx::query_as::<_, Invoice>(
+        "SELECT * FROM billing_invoices WHERE id = $1 AND account_id = $2"
+    )
+    .bind(&invoice_id)
+    .bind(&ctx.account_id)
+    .fetch_optional(&state.db)
+    .await?
+    .ok_or_else(|| SaasError::NotFound("发票不存在".into()))?;
+
+    // 仅已支付的发票可下载 PDF
+    if invoice.status != "paid" {
+        return Err(SaasError::InvalidInput("仅已支付的发票可导出 PDF".into()));
+    }
+
+    // 查询关联支付记录
+    let payments: Vec<Payment> = sqlx::query_as::<_, Payment>(
+        "SELECT * FROM billing_payments WHERE invoice_id = $1"
+    )
+    .bind(&invoice_id)
+    .fetch_all(&state.db)
+    .await?;
+
+    // 构造发票信息（从 invoice metadata 中提取）
+    let info = super::invoice_pdf::InvoiceInfo {
+        title: invoice.metadata.get("invoice_title")
+            .and_then(|v| v.as_str())
+            .unwrap_or("")
+            .to_string(),
+        tax_id: invoice.metadata.get("tax_id")
+            .and_then(|v| v.as_str())
+            .unwrap_or("")
+            .to_string(),
+        email: invoice.metadata.get("email")
+            .and_then(|v| v.as_str())
+            .unwrap_or("")
+            .to_string(),
+        address: invoice.metadata.get("address")
+            .and_then(|v| v.as_str())
+            .unwrap_or("")
+            .to_string(),
+        phone: invoice.metadata.get("phone")
+            .and_then(|v| v.as_str())
+            .unwrap_or("")
+            .to_string(),
+    };
+
+    // 生成 PDF
+    let bytes = super::invoice_pdf::generate_invoice_pdf(&invoice, &payments, &info)
+        .map_err(|e| {
+            tracing::error!("Invoice PDF generation failed: {}", e);
+            SaasError::Internal("PDF 生成失败".into())
+        })?;
+
+    // 返回 PDF 响应
+    Ok(axum::response::Response::builder()
+        .status(200)
+        .header("Content-Type", "application/pdf")
+        .header(
+            "Content-Disposition",
+            format!("attachment; filename=\"invoice-{}.pdf\"", invoice.id),
+        )
+        .body(axum::body::Body::from(bytes))
+        .unwrap())
+}