init: FileDrop phase1 architecture and scaffold

- Rust axum signaling server with WebSocket support
- Lit + TypeScript frontend with Vite
- Redis session storage with TTL
- WebRTC transport and crypto client stubs
- Phase1 architecture plan in plans/
- Deploy directory structure prepared

server/src/session/mod.rs

@@ -0,0 +1,2 @@
+pub mod service;
+pub mod store;