feat: min/max participants — shared ParticipantLimits component

- New ParticipantLimits component in FormField.tsx (reusable)
- Used in both Open Day settings and MC editor — identical layout
- Open Day: event-level min/max (DB migration 15)
- MC: per-event min/max (JSON fields)
- Public: waiting list when full, spots counter, amber success modal

src/app/admin/master-classes/page.tsx

@@ -2,7 +2,7 @@
 
 import { useState, useRef, useEffect, useMemo } from "react";
 import { SectionEditor } from "../_components/SectionEditor";
-import { InputField, TextareaField } from "../_components/FormField";
+import { InputField, TextareaField, ParticipantLimits } from "../_components/FormField";
 import { ArrayEditor } from "../_components/ArrayEditor";
 import { Plus, X, Upload, Loader2, ImageIcon, AlertCircle, Check } from "lucide-react";
 import { adminFetch } from "@/lib/csrf";
@@ -609,6 +609,13 @@ export default function MasterClassesEditorPage() {
                   }
                 />
 
+                <ParticipantLimits
+                  min={item.minParticipants ?? 0}
+                  max={item.maxParticipants ?? 0}
+                  onMinChange={(v) => updateItem({ ...item, minParticipants: v })}
+                  onMaxChange={(v) => updateItem({ ...item, maxParticipants: v })}
+                />
+
               </div>
             )}
             createItem={() => ({