Replace the dual-server architecture (HttpChannel + WasmChannelServer both
competing for port 8080) with a single WebhookServer that composes route
fragments from all sources. Channels define routes but never spawn servers.
- Add WebhookServer struct that collects Router fragments and binds one listener
- Extract routes() from HttpChannel, remove server-spawning from start/shutdown
- Delete WasmChannelServer (keep WasmChannelRouter and route builder)
- Rewire main.rs to compose all webhook routes into one server
Co-Authored-By: Claude Opus 4.6 <[email protected]>
