[ LNVPS/api ] feat: add cryptographically signed LIR agreement generation from subscriptions

- Add new legal.rs module for LIR agreement routes
- Generate signed agreements from subscription data
- Sign agreements with provider's Nostr key (NIP-98)
- Return shareable URL with base64-encoded signed agreement data
- Prevent tampering via Nostr event signatures
- Add nostr crate dependency for signing
- Use Axum v0.8 path parameter syntax ({param})

https://github.com/LNVPS/api/commit/d1b0b2be97d57c5ee2339533e193e43dd73ac123