feat: SSE handler, send y keepalive (issue 0011 fase 2)

functions/infra/sse_keepalive.go

@@ -0,0 +1,34 @@
+package infra
+
+import (
+	"net/http"
+	"time"
+)
+
+// SSEKeepalive envia comentarios SSE periodicos (": keepalive\n\n") al writer
+// hasta que done se cierre. Bloqueante: lanzar como goroutine.
+// Sirve para evitar que proxies/load balancers cierren conexiones inactivas.
+// Si w implementa http.Flusher hace flush tras cada keepalive.
+func SSEKeepalive(w http.ResponseWriter, interval time.Duration, done <-chan struct{}) {
+	if interval <= 0 {
+		interval = 30 * time.Second
+	}
+	ticker := time.NewTicker(interval)
+	defer ticker.Stop()
+
+	flusher, _ := w.(http.Flusher)
+
+	for {
+		select {
+		case <-done:
+			return
+		case <-ticker.C:
+			if _, err := w.Write([]byte(": keepalive\n\n")); err != nil {
+				return
+			}
+			if flusher != nil {
+				flusher.Flush()
+			}
+		}
+	}
+}