feat(server)!: require a logged-in account for every channel request

Drop unauthenticated guests from channels: the allow_guests setting, the
/guests endpoints and the guest seat table are gone, and requests without an
account session get 401 account_required. Share links are unaffected.
Contract version is now 8.0.0.

server/src/WebDrop.Api/ApiContract.cs

@@ -3,7 +3,8 @@ namespace WebDrop.Api;
 public static class ApiContract
 {
     // v5: 频道容量按账号成员计;v7: seats 改名 max_members/members_used/members_full,
-    // 去掉设备名与设备清单接口,访客改走 /guests。设备只是账号的属性。
+    // 去掉设备名与设备清单接口。设备只是账号的属性。
+    // v8: 频道强制登录,去掉未登录访客(allow_guests、/guests、终端席位);便捷分享不受影响。
     // Keep this aligned with contracts/api-contract.json and docs/openapi.yaml.
-    public const string Version = "7.0.0";
+    public const string Version = "8.0.0";
 }