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/service/admin_post_service.go

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