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/Accounts/AccountProfile.cs

@@ -0,0 +1,18 @@
+using System.Text.Json.Serialization;
+
+namespace WebDrop.Core;
+
+public sealed class AccountProfile
+{
+    [JsonPropertyName("username")] public required string Username { get; init; }
+    [JsonPropertyName("displayName")] public required string DisplayName { get; init; }
+    /// <summary>头像(data URL)。资料列表、空间成员等接口不再内联头像(动图可能有几 MB),只告诉有没有,头像另用 /accounts/users/{用户名}/avatar 取</summary>
+    [JsonPropertyName("avatar")] public string? Avatar { get; init; }
+    [JsonPropertyName("hasAvatar")] public bool HasAvatar { get; init; }
+    [JsonPropertyName("modifiedAt")] public required long ModifiedAt { get; init; }
+
+    /// <summary>有损的小图(列表、聊天里默认分发);没有单独上传时用原图。不随 JSON 输出</summary>
+    [JsonIgnore] public string? AvatarThumb { get; init; }
+
+    public AccountProfile WithoutAvatar() => new() { Username = Username, DisplayName = DisplayName, HasAvatar = HasAvatar, ModifiedAt = ModifiedAt };
+}