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.controller.ts

@@ -29,6 +29,19 @@ export class SettingsController {
     return this.settings.updatePolicy(key, body.content);
   }
 
+  // Maps key
+  @Get('maps-key')
+  async getMapsKey() {
+    const api_key = await this.settings.getMapsKey();
+    return { configured: !!api_key, api_key: api_key ?? '' };
+  }
+
+  @Patch('maps')
+  @UseGuards(JwtAuthGuard) @ApiBearerAuth()
+  saveMapsKey(@Body() body: { api_key: string }) {
+    return this.settings.saveMapsKey(body.api_key);
+  }
+
   // Public policy pages
   @Get('policy/:key')
   async getPublicPolicy(@Param('key') key: string, @Res() res: Response) {