perf(host): store avatars as content-addressed files and serve them from disk

Space info now carries avatar_version instead of the inlined thumbnail; the worker fetches the thumbnail once per version. Legacy base64 rows are migrated to files on startup, and the daily backup uploads avatar files.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com> [CI]

server/src/WebDrop.Core/Accounts/AccountProfile.cs

@@ -6,7 +6,7 @@ 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>
+    /// <summary>头像文件名(服务端存的是磁盘文件)。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; }