CUB-136: add SSE endpoint in Go backend

backend/internal/sse/handler.go

@@ -0,0 +1,59 @@
+package sse
+
+import (
+	"net/http"
+)
+
+// Handler is the HTTP handler for the GET /api/events SSE stream.
+// It registers a client with the broadcaster, streams events as they arrive,
+// and unregisters on disconnect.
+type Handler struct {
+	bc *Broadcaster
+}
+
+// NewHandler creates a Handler backed by the given Broadcaster.
+func NewHandler(bc *Broadcaster) *Handler {
+	return &Handler{bc: bc}
+}
+
+// ServeHTTP implements the SSE streaming endpoint.
+// Flusher is required; clients that do not support flushing receive a 501.
+func (h *Handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	flusher, ok := w.(http.Flusher)
+	if !ok {
+		http.Error(w, "streaming not supported", http.StatusNotImplemented)
+		return
+	}
+
+	// SSE-specific headers
+	w.Header().Set("Content-Type", "text/event-stream")
+	w.Header().Set("Cache-Control", "no-cache")
+	w.Header().Set("Connection", "keep-alive")
+	w.Header().Set("X-Accel-Buffering", "no") // Disable nginx buffering
+
+	// Write headers immediately
+	flusher.Flush()
+
+	// Subscribe to the broadcaster
+	ch := h.bc.Subscribe()
+	defer h.bc.Unsubscribe(ch)
+
+	// Use request context for cancellation when the client disconnects.
+	ctx := r.Context()
+
+	for {
+		select {
+		case <-ctx.Done():
+			return
+		case msg, ok := <-ch:
+			if !ok {
+				return
+			}
+			_, err := w.Write([]byte(msg))
+			if err != nil {
+				return
+			}
+			flusher.Flush()
+		}
+	}
+}