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/tests/WebDrop.Api.Tests/ServerFixture.cs

@@ -19,6 +19,7 @@ public sealed class ServerFixture : IAsyncLifetime, IAsyncDisposable
     private readonly string _dir = Path.Combine(Path.GetTempPath(), "webdrop-api-" + Guid.NewGuid().ToString("N"));
     private readonly Dictionary<string, string> _overrides;
 
+    public string DataDir => _dir;
     public WebApplication App { get; private set; } = null!;
     public HttpClient Http { get; private set; } = null!;
     public Uri BaseUri { get; private set; } = null!;