feat: Google Maps API key management in admin settings

- Backend: GET /settings/maps-key (public) + PATCH /settings/maps (protected)
- Admin: sección para guardar/ver estado de la Maps API key
- Flutter web carga la key dinámicamente desde el backend

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

backend/src/settings/settings.service.ts

@@ -41,4 +41,17 @@ export class SettingsService {
     ]);
     return { privacy, terms };
   }
+
+  async getMapsKey(): Promise<string | null> {
+    const setting = await this.prisma.settings.findUnique({ where: { key: 'maps_config' } });
+    return (setting?.value as any)?.api_key ?? null;
+  }
+
+  async saveMapsKey(api_key: string) {
+    return this.prisma.settings.upsert({
+      where: { key: 'maps_config' },
+      create: { key: 'maps_config', value: { api_key } },
+      update: { value: { api_key } },
+    });
+  }
 }