Complete self-serve maker registration, get an API key

Submits the party-key signature(s) over `topology[].hash` from `register/start`; the desk allocates the party, registers it as a node-hosted maker, and returns a fresh `X-API-Key` (`mk_…`) IN CLEARTEXT — this is the only time it is shown; the desk stores only its hash. Store it before doing anything else. Nothing else needs signing before you can quote. Deposits of any token (Canton Coin included) arrive in `GET /wallet/incoming` for you to accept.

curl -X POST "https://example.com/maker/register/complete" \  -H "Content-Type: application/json" \  -d '{    "registrationId": "string",    "signatures": [      "string"    ]  }'

{  "partyId": "string",  "hint": "acme-mm-a1b2c3",  "apiKey": "mk_5c3b8b7e-2f1a-4e9d-9c3a-1a2b3c4d5e6f"}

{  "error": "string",  "code": "INSUFFICIENT_FUNDS"}

{  "error": "string",  "code": "INSUFFICIENT_FUNDS"}

{  "error": "string",  "code": "INSUFFICIENT_FUNDS"}