chore: import MusifyBackend into monorepo (preserving history)

git-subtree-dir: MusifyBackend
git-subtree-mainline: 6829e769ed
git-subtree-split: ed7b00c2cc

MusifyBackend/Application/Contracts/IStreamTicketService.cs

@@ -0,0 +1,9 @@
+namespace Musify.Application.Contracts
+{
+    public interface IStreamTicketService
+    {
+        StreamTicket IssueTicket(long userId, string keyPrefix);
+    }
+
+    public record StreamTicket(string Token, int ExpiresInSeconds);
+}