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/Cargo.toml

@@ -51,6 +51,7 @@ aes-gcm = { workspace = true }
 sha2 = { workspace = true }
 bytes = { workspace = true }
 async-stream = { workspace = true }
+genpdf = "0.2"
 
 [dev-dependencies]
 tempfile = { workspace = true }