feat(server): implement websocket chat backend

app/websocket/manager.py

@@ -0,0 +1,74 @@
+from dataclasses import dataclass
+from typing import Any
+
+from fastapi import WebSocket
+
+
+@dataclass
+class ClientConnection:
+    websocket: WebSocket
+    client_id: str
+
+
+class ConnectionManager:
+    def __init__(self):
+        self.active_connections: dict[str, list[ClientConnection]] = {}
+
+    async def connect(self, websocket: WebSocket, room_id: str, client_id: str):
+        await websocket.accept()
+        self.active_connections.setdefault(room_id, [])
+        self.active_connections[room_id].append(
+            ClientConnection(websocket=websocket, client_id=client_id)
+        )
+
+    def disconnect(self, websocket: WebSocket, room_id: str):
+        if room_id not in self.active_connections:
+            return
+
+        self.active_connections[room_id] = [
+            connection
+            for connection in self.active_connections[room_id]
+            if connection.websocket is not websocket
+        ]
+
+        if not self.active_connections[room_id]:
+            del self.active_connections[room_id]
+
+    async def send_json(self, websocket: WebSocket, payload: dict[str, Any]):
+        await websocket.send_json(payload)
+
+    async def broadcast_json(self, payload: dict[str, Any], room_id: str):
+        if room_id not in self.active_connections:
+            return
+
+        dead_connections: list[WebSocket] = []
+
+        for connection in list(self.active_connections[room_id]):
+            try:
+                await connection.websocket.send_json(payload)
+            except Exception:
+                dead_connections.append(connection.websocket)
+
+        for websocket in dead_connections:
+            self.disconnect(websocket, room_id)
+
+    def room_users(self, room_id: str) -> list[str]:
+        return [
+            connection.client_id
+            for connection in self.active_connections.get(room_id, [])
+        ]
+
+    def rooms_overview(self) -> dict[str, Any]:
+        return {
+            "rooms": [
+                {
+                    "room_id": room_id,
+                    "online_count": len(connections),
+                    "users": [connection.client_id for connection in connections],
+                }
+                for room_id, connections in self.active_connections.items()
+            ]
+        }
+
+
+manager = ConnectionManager()