feat: skin site UUID lookup - auto-fetch UUID from player name

- GET /settings/uuid-lookup?site=&name= : proxy Yggdrasil API
  (POST {site}/api/yggdrasil/api/profiles/minecraft), SSRF guard,
  10s timeout, name regex, UUID formatting (with dashes)
- GET /settings/skin-site : public read of configured skin site
- settings page: 皮肤站地址 config
- bind identity modals (player dashboard + admin users): 获取UUID button,
  auto-fill site from settings, auto-fill UID from lookup
- verified live against littleskin.cn (Steve -> df273bda...)
- API docs updated

public/js/pages/settings-page.js

@@ -14,6 +14,7 @@ const SettingsPage = {
             <div class="form-group"><label>站点地址</label><input id="s-url" value="${U.esc(settings.site_url?.value||'')}"></div></div>
             <div class="grid-2"><div class="form-group"><label>页尾版权</label><input id="s-copyright" value="${U.esc(settings.copyright?.value||'')}" placeholder="&copy; 2024 MC举报系统"></div>
             <div class="form-group"><label>ICP备案号</label><input id="s-icp" value="${U.esc(settings.icp?.value||'')}" placeholder="沪ICP备XXXXXXXX号"></div></div>
+            <div class="form-group"><label>皮肤站地址</label><input id="s-skin-site" value="${U.esc(settings.skin_site?.value||'')}" placeholder="https://littleskin.cn（用于绑定皮肤站身份时自动查询UUID）"></div>
             <button type="submit" class="btn btn-p btn-sm"><i class="fas fa-save"></i> 保存站点设置</button>
           </form>
         </div></div>
@@ -39,6 +40,7 @@ const SettingsPage = {
             site_url: {value: document.getElementById('s-url').value, label:'站点地址'},
             copyright: {value: document.getElementById('s-copyright').value, label:'页尾版权'},
             icp: {value: document.getElementById('s-icp').value, label:'ICP备案号'},
+            skin_site: {value: document.getElementById('s-skin-site').value.trim(), label:'皮肤站地址'},
           });
           alert('已保存');
         } catch (ex) { alert(ex.message); }