fix: auth and ratelimit middleware return JSON errors instead of text/plain

backend/mw/auth.go

@@ -23,7 +23,7 @@ func RequireAuth(next http.Handler) http.Handler {
 	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		authHeader := r.Header.Get("Authorization")
 		if authHeader == "" || !strings.HasPrefix(authHeader, "Bearer ") {
-			http.Error(w, "missing or invalid authorization header", http.StatusUnauthorized)
+			RespondJSON(w, http.StatusUnauthorized, map[string]string{"error": "missing or invalid authorization header"})
 			return
 		}
 
@@ -31,7 +31,7 @@ func RequireAuth(next http.Handler) http.Handler {
 
 		userID, role, jti, err := auth.VerifyToken(tokenString, r.Context())
 		if err != nil {
-			http.Error(w, "invalid token", http.StatusUnauthorized)
+			RespondJSON(w, http.StatusUnauthorized, map[string]string{"error": "invalid token"})
 			return
 		}
 
@@ -73,7 +73,7 @@ func RequireRole(allowedRoles ...string) func(http.Handler) http.Handler {
 		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 			role, ok := r.Context().Value(UserRoleKey).(string)
 			if !ok {
-				http.Error(w, "unauthorized", http.StatusUnauthorized)
+				RespondJSON(w, http.StatusUnauthorized, map[string]string{"error": "unauthorized"})
 				return
 			}
 
@@ -81,7 +81,7 @@ func RequireRole(allowedRoles ...string) func(http.Handler) http.Handler {
 			hasRole := slices.Contains(allowedRoles, role)
 
 			if !hasRole {
-				http.Error(w, "forbidden", http.StatusForbidden)
+				RespondJSON(w, http.StatusForbidden, map[string]string{"error": "forbidden"})
 				return
 			}
 
@@ -102,7 +102,7 @@ func RequireAdmin(next http.Handler) http.Handler {
 	return RequireRole("admin")(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		uid, ok := r.Context().Value(UserIDKey).(string)
 		if !ok || uid == "" {
-			http.Error(w, "Authentication required", http.StatusUnauthorized)
+			RespondJSON(w, http.StatusUnauthorized, map[string]string{"error": "Authentication required"})
 			return
 		}
 		next.ServeHTTP(w, r)

backend/mw/ratelimit.go

@@ -134,7 +134,7 @@ func RateLimit(limit int, window time.Duration) func(http.Handler) http.Handler
 			}
 
 			if !limiter.Allow(ip) {
-				http.Error(w, "Rate limit exceeded", http.StatusTooManyRequests)
+				RespondJSON(w, http.StatusTooManyRequests, map[string]string{"error": "Rate limit exceeded"})
 				return
 			}