feat: set up WebSocket server infrastructure

- Create type definitions for WebSocket messages and client management
- Implement EventEmitter-based WebSocket server with connection handling
- Add message routing and broadcast capabilities for user subscriptions
- Include comprehensive test suite with 4 passing tests
- Support client presence tracking and message queuing

Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

lib/websocket/types.ts

@@ -0,0 +1,43 @@
+export type WebSocketMessageType =
+  | 'highlight:create'
+  | 'highlight:update'
+  | 'highlight:delete'
+  | 'highlight:sync'
+  | 'presence:online'
+  | 'presence:offline'
+  | 'sync:request'
+  | 'sync:response'
+
+export interface WebSocketMessage {
+  type: WebSocketMessageType
+  payload: Record<string, any>
+  timestamp: number
+  clientId: string
+}
+
+export interface SyncRequest {
+  clientId: string
+  lastSyncTime: number
+  userId: string
+}
+
+export interface SyncResponse {
+  highlights: any[]
+  serverTime: number
+  hasMore: boolean
+}
+
+export interface ClientPresence {
+  clientId: string
+  userId: string
+  online: boolean
+  lastSeen: number
+}
+
+export interface WebSocketServerOptions {
+  port: number
+  cors?: {
+    origin: string | string[]
+    credentials: boolean
+  }
+}