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-growth/Cargo.toml

@@ -39,3 +39,9 @@ zclaw-types = { workspace = true }
 
 [dev-dependencies]
 tokio-test = "0.4"
+criterion = { version = "0.5", features = ["html_reports"] }
+
+[[bench]]
+name = "retrieval_bench"
+harness = false
+path = "benches/retrieval_bench.rs"