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>

admin-v2/package.json

@@ -21,6 +21,7 @@
     "react": "^19.2.4",
     "react-dom": "^19.2.4",
     "react-router-dom": "^7.13.2",
+    "recharts": "^3.8.1",
     "zustand": "^5.0.12"
   },
   "devDependencies": {