feat: Migrate from Socket.IO to Server-Sent Events (SSE)

- Replace Socket.IO with SSE for real-time server-to-client communication
- Add SSE service with client management and topic-based subscriptions
- Implement SSE authentication middleware and streaming endpoints
- Update all backend routes to emit SSE events instead of Socket.IO
- Create SSE context provider for frontend with EventSource API
- Update all frontend components to use SSE instead of Socket.IO
- Add comprehensive SSE tests for both backend and frontend
- Remove Socket.IO dependencies and legacy files
- Update documentation to reflect SSE architecture

Benefits:
- Simpler architecture using native browser EventSource API
- Lower bundle size (removed socket.io-client dependency)
- Better compatibility with reverse proxies and load balancers
- Reduced resource usage for Raspberry Pi deployment
- Standard HTTP-based real-time communication

🤖 Generated with [AI Assistant]

Co-Authored-By: AI Assistant <noreply@ai-assistant.com>

backend/server.js

@@ -1,15 +1,14 @@
 require("dotenv").config();
 const express = require("express");
 const couchdbService = require("./services/couchdbService");
+const sseService = require("./services/sseService");
 const cors = require("cors");
 const http = require("http");
-const socketio = require("socket.io");
 const helmet = require("helmet");
 const rateLimit = require("express-rate-limit");
 const mongoSanitize = require("express-mongo-sanitize");
 const xss = require("xss-clean");
 const { errorHandler } = require("./middleware/errorHandler");
-const socketAuth = require("./middleware/socketAuth");
 const requestLogger = require("./middleware/requestLogger");
 const logger = require("./utils/logger");
 const { validateEnv, logEnvConfig } = require("./utils/validateEnv");
@@ -26,13 +25,6 @@ try {
 
 const app = express();
 const server = http.createServer(app);
-const io = socketio(server, {
-  cors: {
-    origin: process.env.FRONTEND_URL || "http://localhost:3000",
-    methods: ["GET", "POST"],
-    credentials: true,
-  },
-});
 const port = process.env.PORT || 5000;
 
 // Trust proxy - required when behind ingress/reverse proxy
@@ -101,34 +93,8 @@ if (process.env.NODE_ENV !== 'test') {
     });
 }
 
-// Socket.IO Authentication Middleware
-io.use(socketAuth);
-
-// Socket.IO Setup with Authentication
-io.on("connection", (socket) => {
-  logger.info(`Socket.IO client connected`, { userId: socket.user.id });
-
-  socket.on("joinEvent", (eventId) => {
-    socket.join(`event_${eventId}`);
-    logger.debug(`User joined event`, { userId: socket.user.id, eventId });
-  });
-
-  socket.on("joinPost", (postId) => {
-    socket.join(`post_${postId}`);
-    logger.debug(`User joined post`, { userId: socket.user.id, postId });
-  });
-
-  socket.on("eventUpdate", (data) => {
-    io.to(`event_${data.eventId}`).emit("update", data.message);
-  });
-
-  socket.on("disconnect", () => {
-    logger.info(`Socket.IO client disconnected`, { userId: socket.user.id });
-  });
-});
-
-// Make io available to routes
-app.set("io", io);
+// Make sse available to routes
+app.set("sse", sseService);
 
 // Routes
 const authRoutes = require("./routes/auth");
@@ -147,6 +113,7 @@ const cacheRoutes = require("./routes/cache");
 const profileRoutes = require("./routes/profile");
 const analyticsRoutes = require("./routes/analytics");
 const leaderboardRoutes = require("./routes/leaderboard");
+const sseRoutes = require("./routes/sse");
 
 // Apply rate limiters
 app.use("/api/auth/register", authLimiter);
@@ -158,15 +125,10 @@ app.get("/api/health", async (req, res) => {
   try {
     const couchdbStatus = await couchdbService.checkConnection();
     
-    // Check Socket.IO status
-    const socketIOStatus = {
-      engine: io.engine ? "running" : "stopped",
-      connectedClients: io.engine ? io.engine.clientsCount : 0,
-      // Get number of connected sockets
-      sockets: io.sockets ? io.sockets.sockets.size : 0
-    };
+    // Get SSE stats
+    const sseStats = sseService.getStats();
     
-    const isHealthy = couchdbStatus && io.engine;
+    const isHealthy = couchdbStatus;
     
     res.status(isHealthy ? 200 : 503).json({
       status: isHealthy ? "healthy" : "degraded",
@@ -174,10 +136,9 @@ app.get("/api/health", async (req, res) => {
       uptime: process.uptime(),
       services: {
         couchdb: couchdbStatus ? "connected" : "disconnected",
-        socketIO: {
-          status: socketIOStatus.engine,
-          connectedClients: socketIOStatus.connectedClients,
-          activeSockets: socketIOStatus.sockets
+        sse: {
+          totalClients: sseStats.totalClients,
+          totalTopics: sseStats.totalTopics
         }
       },
       memory: {
@@ -193,47 +154,13 @@ app.get("/api/health", async (req, res) => {
       uptime: process.uptime(),
       services: {
         couchdb: "disconnected",
-        socketIO: "unknown"
+        sse: "unknown"
       },
       error: error.message,
     });
   }
 });
 
-// Detailed Socket.IO health check endpoint
-app.get("/api/health/socketio", (req, res) => {
-  try {
-    const socketIOInfo = {
-      status: io.engine ? "running" : "stopped",
-      connectedClients: io.engine ? io.engine.clientsCount : 0,
-      activeSockets: io.sockets ? io.sockets.sockets.size : 0,
-      rooms: [],
-      timestamp: new Date().toISOString()
-    };
-
-    // Get list of active rooms (excluding auto-generated socket ID rooms)
-    if (io.sockets && io.sockets.adapter && io.sockets.adapter.rooms) {
-      const rooms = Array.from(io.sockets.adapter.rooms.keys()).filter(room => {
-        // Filter out socket ID rooms (they start with socket ID pattern)
-        return room.startsWith('event_') || room.startsWith('post_');
-      });
-      
-      socketIOInfo.rooms = rooms.map(room => {
-        const roomSize = io.sockets.adapter.rooms.get(room)?.size || 0;
-        return { name: room, members: roomSize };
-      });
-    }
-
-    res.status(200).json(socketIOInfo);
-  } catch (error) {
-    res.status(500).json({
-      status: "error",
-      error: error.message,
-      timestamp: new Date().toISOString()
-    });
-  }
-});
-
 // Routes
 app.use("/api/auth", authRoutes);
 app.use("/api/streets", streetRoutes);
@@ -251,6 +178,7 @@ app.use("/api/cache", cacheRoutes);
 app.use("/api/profile", profileRoutes);
 app.use("/api/analytics", analyticsRoutes);
 app.use("/api/leaderboard", leaderboardRoutes);
+app.use("/api/sse", sseRoutes);
 
 app.get("/", (req, res) => {
   res.send("Street Adoption App Backend");
@@ -267,7 +195,7 @@ if (require.main === module) {
 }
 
 // Export app and server for testing
-module.exports = { app, server, io };
+module.exports = { app, server };
 
 // Graceful shutdown
 process.on("SIGTERM", async () => {