Initial commit: split VNOX monorepo into VNOX-Server

Unified server binary (vnox-serverd) combining:
- Gateway (TCP, auth, channels, sessions, SQLite)
- Voice-node (UDP relay, Opus, jitter buffer)

Standalone gateway (vnox-gateway) and voice-node (vnox-voice-node) preserved as sub-crates.

gateway/src/handler/content/chat.rs

@@ -0,0 +1,66 @@
+use anyhow::Result;
+use tracing::{debug, warn};
+
+use crate::{
+    net::state::{BroadcastMsg, State},
+    proto::{ChatMessagePayload, ErrorCode, PacketId, encode_packet, to_payload},
+};
+
+pub async fn handle(session_id: &str, mut msg: ChatMessagePayload, state: &State) -> Result<()> {
+    let sess = match crate::domain::session::get(&state.sessions, session_id).await {
+        Some(s) => s,
+        None => return Ok(()),
+    };
+
+    if sess.channel_id.as_deref() != Some(&msg.channel_id) {
+        return Ok(());
+    }
+
+    // Rate limit check (token bucket per session).
+    if !state.rate_limiter.try_consume(session_id) {
+        state.metrics.inc(&state.metrics.rate_limited_events);
+        warn!("rate-limited session {session_id}");
+        let _ = state.broadcast.send(BroadcastMsg {
+            channel_id: None,
+            exclude_session: None,
+            target_session_id: Some(session_id.to_string()),
+            data: encode_packet(
+                PacketId::Error,
+                0,
+                &to_payload(&crate::proto::ErrorPayload {
+                    code: ErrorCode::RateLimited as u32,
+                    message: "you are sending messages too quickly".into(),
+                }),
+            ),
+        });
+        return Ok(());
+    }
+
+    msg.sender_id = sess.user_id.clone();
+    if msg.timestamp == 0 {
+        msg.timestamp = now_ms();
+    }
+
+    state.storage.save_message(&msg).await?;
+    state.metrics.inc(&state.metrics.messages_sent);
+
+    let _ = state.broadcast.send(BroadcastMsg {
+        channel_id: Some(msg.channel_id.clone()),
+        exclude_session: Some(session_id.into()),
+        target_session_id: None,
+        data: encode_packet(PacketId::ChatMessage, 0, &to_payload(&msg)),
+    });
+
+    debug!(
+        "chat {} → {}: {}",
+        sess.nickname, msg.channel_id, msg.content
+    );
+    Ok(())
+}
+
+fn now_ms() -> i64 {
+    std::time::SystemTime::now()
+        .duration_since(std::time::UNIX_EPOCH)
+        .unwrap_or_default()
+        .as_millis() as i64
+}