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/routes/events.js

@@ -55,6 +55,15 @@ router.post(
     // Invalidate events cache
     invalidateCacheByPattern('/api/events');
 
+    // Emit SSE event for new event
+    const sse = req.app.get("sse");
+    if (sse) {
+      sse.broadcastToTopic("events", "eventUpdate", {
+        type: "new_event",
+        event,
+      });
+    }
+
     res.json(event);
   }),
 );
@@ -120,6 +129,16 @@ router.put(
     // Check and award badges
     await couchdbService.checkAndAwardBadges(userId, updatedUser.points);
 
+    // Emit SSE event for RSVP
+    const sse = req.app.get("sse");
+    if (sse) {
+      sse.broadcastToTopic(`event_${eventId}`, "eventUpdate", {
+        type: "participants_updated",
+        eventId,
+        participants: updatedEvent.participants,
+      });
+    }
+
     res.json({
       participants: updatedEvent.participants,
       pointsAwarded: 15,
@@ -172,6 +191,16 @@ router.put(
     }
 
     const updatedEvent = await Event.update(req.params.id, updateData);
+
+    // Emit SSE event for event update
+    const sse = req.app.get("sse");
+    if (sse) {
+      sse.broadcastToTopic(`event_${req.params.id}`, "eventUpdate", {
+        type: "event_updated",
+        event: updatedEvent,
+      });
+    }
+
     res.json(updatedEvent);
   })
 );
@@ -244,6 +273,16 @@ router.delete(
     }
 
     await Event.delete(req.params.id);
+
+    // Emit SSE event for event deletion
+    const sse = req.app.get("sse");
+    if (sse) {
+      sse.broadcastToTopic(`event_${req.params.id}`, "eventUpdate", {
+        type: "event_deleted",
+        eventId: req.params.id,
+      });
+    }
+
     res.json({ msg: "Event deleted successfully" });
   })
 );