Graceful shutdown should cancel in-flight payment verification

[ContextVM-org]

When a priced request arrives, the payment middlewares spawn detached background tasks that poll processor.verifyPayment until the payment verifies or verifyTimeoutMs (default ≤ 5 min) elapses:

• transparent: src/payments/server-payments.ts — the inFlight verify loop inside request processing
• explicit gating: src/payments/server-explicit-gating.ts — the detached verify task after -32042

NostrServerTransport.close() clears the correlation store, session store, and stream factories — but nothing signals these tasks to stop. rs-sdk cancels them via a cancellation token parented to server shutdown.

Observed effects (worst first)

1. Paid tool executes after shutdown began (transparent mode): a verify that resolves after close() proceeds to forward the invocation to the underlying MCP handler and publish payment_accepted. In-flight-at-close requests only, but it breaks rolling-deploy/graceful-shutdown expectations.
2. Process exit delayed: the poll timers (withTimeout's setTimeout) keep the event loop alive up to verifyTimeoutMs after close() — hangs shutdown scripts and test teardown.
3. Wasted external polls: continues hitting the payment processor's API (LNbits/NWC/…) for up to 5 min after the server is gone.
4. Grant written to dead state (explicit gating): verify success post-close writes into an AuthorizationStore nobody reads. Harmless today (new withServerPayments = new store), noting for completeness.

Bounds / severity

Not a money bug: no double-charge, no unbounded growth, no cross-restart state leak. Everything is capped by pending count (≤ maxPendingPayments) and verifyTimeoutMs. Hygiene + graceful-shutdown correctness.

Proposed fix shape

• withServerPayments creates an AbortController shared by both middlewares.
• NostrServerTransport exposes a middleware-observable close hook (none exists for this purpose today).
• Verify/poll loops check signal.aborted per iteration and bail (explicit: skip the grant; transparent: skip the forward — kills effect 1).
• On abort, treat as verify-failure cleanup (clear pending) so state stays coherent.

~30 lines across 4 files (server-payments.ts, server-explicit-gating.ts, server-transport-payments.ts, nostr-server-transport.ts), plus a test that closes mid-verify and asserts no forward and no lingering timer.