feat(server): add general-purpose inbound middleware seam (CEP-8)

[harsh04044]

Part of #100

Adds the inbound middleware seam to NostrServerTransport, the port of ts-sdk's addInboundMiddleware. This is the hook CEP-8 payment gating will register on. Nothing registers on it yet, so on its own the change is behavior-preserving.

With an empty chain the inbound path is unchanged: dispatch_inbound forwards the request to the worker exactly as before, same IncomingRequest. The existing CEP-22 and CEP-41 suites pass untouched, which was the bar for this one.

What's here:

• middleware.rs: the InboundMiddleware trait, the Next continuation, InboundContext, and the dispatcher/runner.
• add_inbound_middleware() on the transport, wired into the event loop at both delivery points (normal requests and reassembled CEP-22 oversized ones).
• PaymentInteractionMode in core::types.

Note:

• The chain runs on its own task rather than inline, so a middleware that blocks (payment verification, later) doesn't stall event ingestion. Empty chain skips the spawn.
• Route cleanup keys off whether the request actually reached the worker, not off the middleware's return value, so a dropped request frees its correlation route and a delivered one keeps it even if a middleware returns the wrong bool.

New unit tests cover forward, drop, transform, ordering, panic, and notification handling.