Add support for subscription as NFTs

[dennj]

1. The facilitator should be able to release an NFT as confirmation of a payment for a subscription.
2. The wallets should be able to purchase NFTs and display all subscriptions, along with related information.
3. The wallet should be able to use the NFT to access a service.
4. The facilitator should be able to verify if an NFT subscription is valid.

[douglasborthwick-crypto]

For requirement #4 (facilitator verifies if an NFT subscription is valid), InsumerAPI handles this with a single call:

POST /v1/attest
{
  "wallet": "0xSUBSCRIBER",
  "conditions": [
    { "type": "nft_ownership", "chainId": 8453, "contractAddress": "0xSUBSCRIPTION_NFT" }
  ]
}
→ { "pass": true, "sig": "ECDSA-P256-signature" }

Returns a signed boolean: does this wallet hold the subscription NFT? Works across 31 chains. The ECDSA signature means the facilitator can verify the response cryptographically without trusting the caller.

Since this is an MCP project, the mcp-server-insumer package (16 tools, stdio transport) gives agents direct access to verify-holdings and check-nft as callable tools. An agent could verify a subscription NFT before granting access to the weather MCP service, with no custom verification code needed.

Free tier: 100 verifications. OpenAPI spec.