protobuf migration: replace JSON with prost-generated wire format

- Expanded protocol/lnex.proto with full message set for all packet types
- Added prost + prost-build to gateway build pipeline
- Refactored all handlers and framing to use prost encode/decode
- Removed serde payload structs (now generated from .proto)
- Server starts and admin endpoint responds

gateway/Cargo.toml

@@ -38,9 +38,13 @@ axum  = { version = "0.8", features = ["ws"] }
 tower = "0.5"
 sqlx     = { version = "0.9", features = ["sqlite", "runtime-tokio", "tls-rustls"] }
 bitflags = "2"
+prost = "0.13"
 
 # TLS 1.3 support
 rustls-pki-types = "1"
 rustls = "0.23"
 tokio-rustls = "0.26"
 rcgen = "0.13"
+
+[build-dependencies]
+prost-build = "0.13"