Gate scoped BTCPay connect to sandbox + non-mainnet

Slices 3-4 of agent-payment-connect: a scoped key carrying the a-la-carte
payment_providers:write scope may connect a BTCPay provider, but only on a
sandbox daemon (KEYSAT_SANDBOX_MODE) and only for a non-mainnet
(regtest/testnet/signet) store. Master may connect any network; disconnect and
production/mainnet reconnect stay master-only. A credential that can repoint
settlement is a fund-redirection key, so the gate is deliberately narrow and
fails closed.

- require_provider_connect: outer gate (sandbox flag) at start_connect
- btcpay/network.rs classify_address_network + client::fetch_onchain_network:
  resolve the store network at finish_connect, fail-closed to mainnet on any
  ambiguity (no on-chain method, non-2xx, non-JSON, unknown prefix), before any
  webhook/persist side effect
- initiator carried across the OAuth round-trip via btcpay_authorize_state
  (migration 0025: scoped_initiator + initiator_actor_hash); scoped connects
  are audited
- the GET callback now returns the error's HTTP status (was a misleading 200 on
  a denied connect)
- openapi.rs documents the BTCPay connect/callback/status/disconnect paths and
  the key-creation scopes field

Validated end-to-end against a live regtest BTCPay. Full suite green; adds gate
+ network unit/integration tests.

licensing-service/migrations/0025_authorize_state_scoped_initiator.sql

@@ -0,0 +1,28 @@
+-- Carry the connect *initiator* through the BTCPay OAuth round trip.
+--
+-- agent-payment-connect (plans/agent-payment-connect-scope.md): a scoped key
+-- bearing `payment_providers:write` may start a BTCPay connect, but only on a
+-- sandbox daemon (outer gate) AND only for a non-mainnet store (inner gate).
+-- The inner gate can only be evaluated at callback time — that's the first
+-- moment we know the store and can resolve its network. So the connect handler
+-- must remember, across the operator's browser round-trip to BTCPay, whether
+-- the initiator was the master key (may connect any network) or a scoped key
+-- (restricted to non-mainnet).
+--
+-- `scoped_initiator`: 0 = master (no network restriction), 1 = scoped key
+--   (callback enforces non-mainnet, fail-closed). Default 0 keeps any in-flight
+--   pre-upgrade state token behaving as a master connect (the only kind that
+--   existed before this migration).
+-- `initiator_actor_hash`: sha256 of the initiating credential, so the callback
+--   can write an audit row attributing the scoped connect without a header.
+--
+-- Additive, one-way (consistent with 0020-0022). The table is also pruned by
+-- timestamp, so any pre-migration rows expire within 30 minutes regardless.
+
+PRAGMA foreign_keys = ON;
+
+ALTER TABLE btcpay_authorize_state
+    ADD COLUMN scoped_initiator INTEGER NOT NULL DEFAULT 0;
+
+ALTER TABLE btcpay_authorize_state
+    ADD COLUMN initiator_actor_hash TEXT;