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/src/proto/crypto/cipher.rs

@@ -1,6 +1,6 @@
 use chacha20poly1305::{
-    ChaCha20Poly1305, Key, KeyInit, Nonce,
     aead::{Aead, Payload},
+    ChaCha20Poly1305, Key, KeyInit, Nonce,
 };
 
 pub(super) fn make_nonce(cid: &[u8; 8], seq: u64) -> [u8; 12] {