fix(handler): adjust SSE heartbeat interval and cache control header

- Updated Cache-Control header to include 'no-transform'.
- Reduced heartbeat interval from 25 seconds to 10 seconds to accommodate common proxy idle timeouts.
- Added comments for clarity on keepalive handling for SSE intermediaries and proxies.

internal/handler/message_handler.go

@@ -65,7 +65,7 @@ func (h *MessageHandler) HandleSSE(c *gin.Context) {
 	}
 
 	w.Header().Set("Content-Type", "text/event-stream")
-	w.Header().Set("Cache-Control", "no-cache")
+	w.Header().Set("Cache-Control", "no-cache, no-transform")
 	w.Header().Set("Connection", "keep-alive")
 	w.Header().Set("X-Accel-Buffering", "no")
 	c.Status(http.StatusOK)
@@ -89,7 +89,8 @@ func (h *MessageHandler) HandleSSE(c *gin.Context) {
 		}
 	}
 
-	heartbeat := time.NewTicker(25 * time.Second)
+	// Use a shorter heartbeat interval to survive common 50-60s proxy idle timeout.
+	heartbeat := time.NewTicker(10 * time.Second)
 	defer heartbeat.Stop()
 
 	for {
@@ -101,6 +102,11 @@ func (h *MessageHandler) HandleSSE(c *gin.Context) {
 				return
 			}
 		case <-heartbeat.C:
+			// Comment keepalive line for SSE intermediaries/proxies.
+			if _, err := fmt.Fprint(w, ": keepalive\n\n"); err != nil {
+				return
+			}
+			flusher.Flush()
 			if _, err := fmt.Fprint(w, "event: heartbeat\ndata: {}\n\n"); err != nil {
 				return
 			}

internal/service/admin_post_service.go

@@ -1,4 +1,4 @@
-package service
+package service
 
 import (
 	"context"