feat(server): split file storage into a separate file service with tickets, receipts and WebDAV backup

- new projects WebDrop.FileApi (file service), WebDrop.FileProtocol (shared tickets and receipts) and WebDrop.Sync (WebDAV)
- file content of any size goes to the file service; chat, avatars and space structure stay on the main backend
- tickets issued by the main backend, receipts by the file service, deletion queue and daily reconciliation
- daily database snapshot and file sync to external storage (OpenList/WebDAV) with a 30 day retention
- reorganize Core, Storage and Api into feature folders (namespaces unchanged)
- web client uploads file frames to the file service and reads them with tickets

server/src/WebDrop.Core/Channels/ChannelPolicy.cs

@@ -0,0 +1,26 @@
+namespace WebDrop.Core;
+
+/// <summary>谁能做什么。规则只写在这里,服务和测试都调用它。</summary>
+public static class ChannelPolicy
+{
+    public static bool IsOwner(Principal p) => p.Role == Role.Owner;
+
+    public static bool CanRead(Principal p) => p.Role >= Role.Read;
+
+    /// <summary>成员写入需要频道允许上传,且服务端持有写令牌哈希。</summary>
+    public static bool CanWrite(Channel channel, Principal p) =>
+        p.Role == Role.Owner
+        || (p.Role == Role.Write && channel.Settings.MembersCanUpload && channel.WriteHash is not null);
+
+    public static bool CanDeleteItem(Channel channel, Item item, Principal p)
+    {
+        if (p.Role == Role.Owner) return true;
+        if (p.Role < Role.Read) return false;
+        return channel.Settings.DeletePolicy switch
+        {
+            DeletePolicy.Anyone => true,
+            DeletePolicy.Own => item.AuthorAccount == p.AccountName,
+            _ => false,
+        };
+    }
+}