Add SubscribeEvents server-streaming RPC

Implement the SubscribeEvents RPC end-to-end: a Stream variant in
GrpcBody that delivers multiple gRPC-framed messages from an mpsc
channel, a handler in service.rs that bridges the broadcast channel
to the streaming response, a client EventStream type that reads
gRPC frames incrementally from the HTTP/2 body, and e2e test
assertions that verify payment events arrive via the stream.

Also regenerates proto code to include SubscribeEventsRequest.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: benthecarman