Link Shortener — people sign in; agents pay 0.01 pathUSD

People:
- GET /register and GET /login require a @stripe.com email
- GET /links lists the signed-in user's links
- POST /api/me/links creates a link for that account (optional form field slug, e.g. workshop). Humans do not pay.
- POST /api/me/links/{code} deletes a link for that account

Agents:
- POST /api/shorten body {"url":"https://example.com"} — 0.01 Tempo testnet pathUSD, random 6-char code
- POST /api/shorten/custom body {"url":"...","slug":"workshop"} — 0.01 Tempo mainnet pathUSD (real stablecoin)
- DELETE /api/links/{code} — testnet, random codes only
- DELETE /api/shorten/custom/{slug} — mainnet, named links only
- GET /openapi.json discovery
- GET /{slug} redirect

Use:
npx mppx --network testnet -X POST --json-body '{"url":"https://example.com"}' <origin>/api/shorten
npx mppx --network mainnet -X POST --json-body '{"url":"https://example.com","slug":"workshop"}' <origin>/api/shorten/custom
npx mppx --network testnet -X DELETE <origin>/api/links/<code>
npx mppx --network mainnet -X DELETE <origin>/api/shorten/custom/<slug>