Re-drive the DevNet receive preapproval

Manual recovery for the auto-preapproval attempted at `register/complete`: on DevNet a party needs a `TransferPreapproval` before it can receive CC/cBTC. Idempotent (a no-op if one already exists) and a no-op entirely on the sandbox (MockRegistry needs none). Best-effort — always answers `{ok: true}` even if the underlying preapproval attempt failed; check `GET /wallet/incoming` / `GET /wallet/holdings` to confirm funds actually arrived.

curl -X POST "https://example.com/maker/preapproval"

{  "ok": true}

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