feat(verify): delegated settlement verification endpoint

Add POST /api/v1/verify, the server side of moon.vote's delegated
settlement check: confirm a specific tx_ref paid at least the expected
base-unit amount to dest_address, confirmed to this backend's per-asset
depth.

- UTXO (btc/ltc): mirror ElectrumClient semantics; consult get_history
  first to distinguish not_found from a node error, then attribute the
  exact amount via VerboseTransaction::total_received_at.
- CryptoNote (xmr/wow): require a private view_key, scan via
  get_address_txs, match tx_ref, gate on mempool/height and the daemon
  confirmation depth (XMR 10, WOW 4).
- grin: deferred (reports not_found with a warn).

Business outcomes (incl. failures) are always 200 with a fixed reason
vocabulary; only infrastructure failures return non-2xx AppError so
moon.vote retries. Unauthenticated by design (nginx allow-list gates it);
no utoipa/openapi wiring (server-to-server, not in the wallet client).

src/api/mod.rs

@@ -13,5 +13,6 @@ pub mod premium;
 pub mod prices;
 pub mod tips;
 pub mod users;
+pub mod verify;
 pub mod wallet;
 pub mod website;