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/Models/ShareTokenMaterial.cs

@@ -0,0 +1,13 @@
+namespace Jellyfin.Plugin.ShareLinks.Models;
+
+/// <summary>
+/// A freshly generated share token and its persisted hash.
+/// </summary>
+public sealed class ShareTokenMaterial
+{
+    /// <summary>Gets or sets the raw token returned once to the caller.</summary>
+    public string Token { get; set; } = string.Empty;
+
+    /// <summary>Gets or sets the HMAC hash stored durably.</summary>
+    public string TokenHash { get; set; } = string.Empty;
+}