Requests the implementation of 'verify_payment' in the x402 middleware to perform on-chain verification of subscription channels using '@stellar/stellar-sdk', ensuring that requests with payment headers are only accepted if the subscription is active and has remaining balance.
## Description The middleware currently accepts any request with payment headers without actually verifying the subscription channel on-chain. The TODO at line 40 needs implementation. ## What needs to happen - Use `@stellar/stellar-sdk` to make a read-only Soroban call to `verify_payment` - Pass the subscriber's stellar address and service address - Check that `active === true` and `remaining > 0` - Return 402 if verification fails - Cache verification results briefly to reduce RPC calls ## Files - `src/middleware/x402-subscription.ts` ## Acceptance Criteria - [ ] Middleware calls Soroban RPC to verify channel - [ ] Invalid/expired channels are rejected with 402 - [ ] Active channels pass through to the handler