Add UserId newtype and WebSocket ping keep-alive

Replace raw String user IDs with a UserId newtype in domain for
type safety, and add 30s ping/pong to prevent idle WebSocket
disconnects.

crates/application/src/soldiers/disconnect.rs

@@ -1,8 +1,8 @@
-use domain::BroadcastEvent;
+use domain::{BroadcastEvent, UserId};
 
 use crate::AppState;
 
-pub fn execute(state: &AppState, user_id: &str) -> usize {
+pub fn execute(state: &AppState, user_id: &UserId) -> usize {
     state.cooldowns().remove(user_id);
     let count = state.soldiers().remove(user_id);
     state