ShareLinks plugin: guest share links for Jellyfin

Includes fix for redemption failing with DbUpdateConcurrencyException:
change the guest password before applying the user policy, since
UpdatePolicyAsync bumps the user's EF concurrency token and a stale
instance then breaks ChangePassword.

Jellyfin.Plugin.ShareLinks/Services/IShareLinkCleanupService.cs

@@ -0,0 +1,13 @@
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace Jellyfin.Plugin.ShareLinks.Services;
+
+/// <summary>
+/// Cleanup seam for later workers. The initial implementation is a no-op.
+/// </summary>
+public interface IShareLinkCleanupService
+{
+    /// <summary>Runs one cleanup pass.</summary>
+    Task CleanupAsync(CancellationToken cancellationToken);
+}