Purchase brief

How bots find a paid article

1. Free discovery: GET /api/v1/catalog
2. Paid URL: GET /api/v1/papers/{id} ($0.005 USDC on Base)
3. Unpaid → HTTP 402
4. Sign USDC authorization (MetaMask popup) → 200 JSON

Checklist: MetaMask unlocked · network Base · wallet holds USDC (not only ETH) · amount ≥ $0.005