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/domain/src/lib.rs

@@ -8,6 +8,6 @@ pub use canvas::Canvas;
 pub use errors::DomainError;
 pub use events::BroadcastEvent;
 pub use ports::BroadcastSubscription;
-pub use value_objects::{Color, PixelUpdate, Position};
+pub use value_objects::{Color, PixelUpdate, Position, UserId};
 
 pub const COOLDOWN_MESSAGE: &str = "You can only place one pixel per minute";

crates/domain/src/value_objects.rs

@@ -1,5 +1,24 @@
 use std::fmt;
 
+#[derive(Debug, Clone, PartialEq, Eq, Hash)]
+pub struct UserId(String);
+
+impl UserId {
+    pub fn new(id: String) -> Self {
+        Self(id)
+    }
+
+    pub fn as_str(&self) -> &str {
+        &self.0
+    }
+}
+
+impl fmt::Display for UserId {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.write_str(&self.0)
+    }
+}
+
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 #[repr(transparent)]
 pub struct Color(u32);