Make Ollama prompt editable in admin AI settings

- Add prompt column to ai_settings (migration)
- AiSetting: DEFAULT_PROMPT constant + getActivePrompt() (falls back to default if empty)
- ProcessNailPolishImage: reads prompt from DB instead of hardcoded
- Admin view: textarea to edit prompt, shows active prompt, reset-to-default link

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/Http/Controllers/Admin/AiSettingController.php

@@ -19,11 +19,13 @@ class AiSettingController extends Controller
         $request->validate([
             'ollama_url'   => 'required|url|max:255',
             'ollama_model' => 'required|string|max:100',
+            'prompt'       => 'nullable|string',
         ]);
 
         $ai = AiSetting::get();
         $ai->ollama_url   = rtrim($request->ollama_url, '/');
         $ai->ollama_model = $request->ollama_model;
+        $ai->prompt       = $request->filled('prompt') ? $request->prompt : null;
         $ai->save();
 
         return redirect()->route('admin.ai.show')->with('success', 'KI-Einstellungen gespeichert.');