feat(server): implement sealed share lifecycle

Signed-off-by: JustMe <mr.toing@outlook.com>

server/src/WebDrop.Core/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.AuthorHash.AsSpan().SequenceEqual(p.EndpointHash),
+            _ => false,
+        };
+    }
+}