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/WebDrop.slnx

@@ -2,8 +2,11 @@
   <Folder Name="/src/">
     <Project Path="src/WebDrop.Api/WebDrop.Api.csproj" />
     <Project Path="src/WebDrop.Core/WebDrop.Core.csproj" />
+    <Project Path="src/WebDrop.FileApi/WebDrop.FileApi.csproj" />
+    <Project Path="src/WebDrop.FileProtocol/WebDrop.FileProtocol.csproj" />
     <Project Path="src/WebDrop.Frontend/WebDrop.Frontend.csproj" />
     <Project Path="src/WebDrop.Storage/WebDrop.Storage.csproj" />
+    <Project Path="src/WebDrop.Sync/WebDrop.Sync.csproj" />
   </Folder>
   <Folder Name="/tests/">
     <Project Path="tests/WebDrop.Api.Tests/WebDrop.Api.Tests.csproj" />