fix: prevent internal error details leaking in HTTP responses

Replace err.Error() concatenation in JSON decode error responses with fixed 'invalid request body' message across 5 locations in custom_services.go, discount_campaigns.go, and services.go.

backend/handlers/admin/custom_services.go

@@ -207,7 +207,7 @@ func GetCustomServices(w http.ResponseWriter, r *http.Request) {
 func CreateCustomService(w http.ResponseWriter, r *http.Request) {
 	var req CreateCustomServiceRequest
 	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
-		http.Error(w, "Invalid JSON: "+err.Error(), http.StatusBadRequest)
+		http.Error(w, "invalid request body", http.StatusBadRequest)
 		return
 	}
 	if err := validators.Validate.Struct(&req); err != nil {
@@ -326,7 +326,7 @@ func UpdateCustomService(w http.ResponseWriter, r *http.Request) {
 
 	var req UpdateCustomServiceRequest
 	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
-		http.Error(w, "Invalid JSON: "+err.Error(), http.StatusBadRequest)
+		http.Error(w, "invalid request body", http.StatusBadRequest)
 		return
 	}
 	if err := validators.Validate.Struct(&req); err != nil {

backend/handlers/admin/discount_campaigns.go

@@ -211,7 +211,7 @@ func GetDiscountCampaigns(w http.ResponseWriter, r *http.Request) {
 func CreateDiscountCampaign(w http.ResponseWriter, r *http.Request) {
 	var req CreateCampaignRequest
 	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
-		http.Error(w, "Invalid JSON: "+err.Error(), http.StatusBadRequest)
+		http.Error(w, "invalid request body", http.StatusBadRequest)
 		return
 	}
 
@@ -417,7 +417,7 @@ func UpdateDiscountCampaign(w http.ResponseWriter, r *http.Request) {
 
 	var req UpdateCampaignRequest
 	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
-		http.Error(w, "Invalid JSON: "+err.Error(), http.StatusBadRequest)
+		http.Error(w, "invalid request body", http.StatusBadRequest)
 		return
 	}