feat: harden SSE endpoint with typed events and Last-Event-ID

- Added monotonic event ID (Last-Event-ID) for reconnection support
- Events now emit typed: 'event: camera_status' for client-side filtering
- Initial connection event sends 'event: connected' with ID
- Reconnection acknowledgment via Last-Event-ID header
- CORS preflight (OPTIONS) handler
- Access-Control-Allow-Headers: Last-Event-ID
- Initial heartbeat shortened to 15s for faster detect (30s for steady)

Closes CUB-233.

internal/events/sse.go

@@ -7,13 +7,15 @@ import (
 	"log"
 	"net/http"
 	"sync"
+	"sync/atomic"
 	"time"
 )
 
 // Hub manages SSE client connections and event broadcasting.
 type Hub struct {
-	mu      sync.RWMutex
+	mu       sync.RWMutex
-	clients map[*Client]bool
+	clients  map[*Client]bool
+	eventSeq atomic.Int64 // monotonic event ID for Last-Event-ID
 }
 
 // NewHub creates a new SSE hub.
@@ -58,6 +60,13 @@ func (h *Hub) serveSSE(w http.ResponseWriter, r *http.Request) {
 	w.Header().Set("Cache-Control", "no-cache")
 	w.Header().Set("Connection", "keep-alive")
 	w.Header().Set("Access-Control-Allow-Origin", "*")
+	w.Header().Set("Access-Control-Allow-Headers", "Last-Event-ID")
+
+	// Handle preflight
+	if r.Method == http.MethodOptions {
+		w.WriteHeader(http.StatusOK)
+		return
+	}
 
 	// Get flusher
 	flusher, ok := w.(http.Flusher)
@@ -85,12 +94,21 @@ func (h *Hub) serveSSE(w http.ResponseWriter, r *http.Request) {
 		client.Close()
 	}()
 
+	// Acknowledge Last-Event-ID if sent by client on reconnect
+	if lastEventID := r.Header.Get("Last-Event-ID"); lastEventID != "" {
+		fmt.Fprintf(w, "id: %s\nevent: reconnected\ndata: {\"type\":\"reconnected\",\"last_event_id\":\"%s\"}\n\n", lastEventID, lastEventID)
+		flusher.Flush()
+	}
+
 	// Send initial connection event
-	data, _ := json.Marshal(map[string]string{
+	seq := h.eventSeq.Add(1)
+	data, _ := json.Marshal(map[string]interface{}{
 		"type": "connected",
+		"id":   seq,
 		"ts":   time.Now().Format(time.RFC3339),
 	})
-	if !client.Write(data) {
+	eventLine := fmt.Sprintf("id: %d\nevent: connected\ndata: %s\n\n", seq, string(data))
+	if !client.Write([]byte(eventLine)) {
 		return // client disconnected
 	}
 
@@ -115,13 +133,18 @@ func (h *Hub) serveSSE(w http.ResponseWriter, r *http.Request) {
 	}
 }
 
-// Broadcast sends an event to all connected clients.
+// Broadcast sends a typed SSE event to all connected clients.
+// eventType becomes the "event:" field, enabling client-side filtering.
+// Each event gets a monotonic ID for Last-Event-ID reconnection support.
 func (h *Hub) Broadcast(eventType string, payload interface{}) {
 	h.mu.RLock()
 	defer h.mu.RUnlock()
 
+	seq := h.eventSeq.Add(1)
+
 	event := map[string]interface{}{
 		"type":    eventType,
+		"id":      seq,
 		"ts":      time.Now().Format(time.RFC3339),
 		"payload": payload,
 	}
@@ -132,8 +155,10 @@ func (h *Hub) Broadcast(eventType string, payload interface{}) {
 		return
 	}
 
+	eventLine := fmt.Sprintf("id: %d\nevent: %s\ndata: %s\n\n", seq, eventType, string(data))
+
 	for client := range h.clients {
-		if !client.Write(data) {
+		if !client.Write([]byte(eventLine)) {
 			log.Println("SSE client buffer full, dropping event")
 		}
 	}