feat: add web gateway hot reload and polling state sync (#1684)

* feat(gateway): support hot reload and empty startup

- extract gateway runtime into pkg/gateway
- add gateway.hot_reload config with default and example values
- allow starting the gateway without a default model via --allow-empty
- stop treating missing enabled channels as a startup error
- update related tests

* feat: replace gateway SSE updates with polling-based state sync

- remove gateway SSE broadcasting and event endpoint
- add polling-based gateway status refresh with stopping state handling
- detect when gateway restart is required after default model changes
- resolve gateway health and websocket proxy targets from configured host
- update gateway UI labels and add backend/frontend test coverage

web/backend/middleware/middleware.go

@@ -4,16 +4,14 @@ import (
 	"log"
 	"net/http"
 	"runtime/debug"
-	"strings"
 	"time"
 )
 
 // JSONContentType sets the Content-Type header to application/json for
 // API requests handled by the wrapped handler.
-// SSE endpoints (text/event-stream) are excluded.
 func JSONContentType(next http.Handler) http.Handler {
 	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
-		if strings.HasPrefix(r.URL.Path, "/api/") && !strings.HasSuffix(r.URL.Path, "/events") {
+		if len(r.URL.Path) >= 5 && r.URL.Path[:5] == "/api/" {
 			w.Header().Set("Content-Type", "application/json")
 		}
 		next.ServeHTTP(w, r)
@@ -32,7 +30,6 @@ func (rr *responseRecorder) WriteHeader(code int) {
 }
 
 // Flush delegates to the underlying ResponseWriter if it implements http.Flusher.
-// This is required for SSE (Server-Sent Events) to work through the middleware.
 func (rr *responseRecorder) Flush() {
 	if f, ok := rr.ResponseWriter.(http.Flusher); ok {
 		f.Flush()