Implement Watched Together shared viewing accounts

Replaces the plugin template with a working plugin that lets several
users share one viewing account while keeping their individual watched
lists accurate.

Three pieces:

- Auto-creating groups. Logging in as "alice+bob" with any named
  member's own password provisions the shared account and signs you in.
  Verified against 10.11.5: AuthenticateUser offers unmatched usernames
  to every enabled provider and re-queries afterwards, which is the hook
  this relies on. Gated on a real member password so knowing two
  usernames is not enough to create an account.

- Multi-password authentication. IRequiresResolvedUser hands us the
  resolved shared account; each member's live stored hash is checked via
  ICryptoProvider.Verify. Deliberately avoids re-entering
  UserManager.AuthenticateUser, which would trip every member's
  failed-attempt counter whenever a different member's password matched.

- One-way played-state sync. Shared account to members only, filtered to
  PlaybackFinished/TogglePlayed/Import so playback progress ticks are
  ignored. No loop guard needed: member writes carry a non-shared id.

Membership is stored as user IDs rather than re-parsed from the username,
so shared accounts can be renamed freely. The +/name collision resolves
itself because Jellyfin only consults the plugin when no local user
matches the typed name.

Targets Jellyfin 10.11.x / net9.0. Adds Gitea CI (test, build, release),
a builder image, and 34 tests covering the auth and sync rules.

Jellyfin.Plugin.WatchedTogether/Plugin.cs

@@ -0,0 +1,56 @@
+using System;
+using System.Collections.Generic;
+using System.Globalization;
+using Jellyfin.Plugin.WatchedTogether.Configuration;
+using MediaBrowser.Common.Configuration;
+using MediaBrowser.Common.Plugins;
+using MediaBrowser.Model.Plugins;
+using MediaBrowser.Model.Serialization;
+
+namespace Jellyfin.Plugin.WatchedTogether;
+
+/// <summary>
+/// The Watched Together plugin: shared viewing accounts whose watched state flows back to each
+/// member's own account.
+/// </summary>
+public class Plugin : BasePlugin<PluginConfiguration>, IHasWebPages
+{
+    /// <summary>
+    /// Initializes a new instance of the <see cref="Plugin"/> class.
+    /// </summary>
+    /// <param name="applicationPaths">Instance of the <see cref="IApplicationPaths"/> interface.</param>
+    /// <param name="xmlSerializer">Instance of the <see cref="IXmlSerializer"/> interface.</param>
+    public Plugin(IApplicationPaths applicationPaths, IXmlSerializer xmlSerializer)
+        : base(applicationPaths, xmlSerializer)
+    {
+        Instance = this;
+    }
+
+    /// <inheritdoc />
+    public override string Name => "Watched Together";
+
+    /// <inheritdoc />
+    public override Guid Id => Guid.Parse("aa3288a0-e8c1-43e2-8045-8c3411142a5b");
+
+    /// <inheritdoc />
+    public override string Description =>
+        "Lets several users share one viewing account, with watched state syncing back to each member.";
+
+    /// <summary>
+    /// Gets the current plugin instance.
+    /// </summary>
+    public static Plugin? Instance { get; private set; }
+
+    /// <inheritdoc />
+    public IEnumerable<PluginPageInfo> GetPages()
+    {
+        return
+        [
+            new PluginPageInfo
+            {
+                Name = Name,
+                EmbeddedResourcePath = string.Format(CultureInfo.InvariantCulture, "{0}.Configuration.configPage.html", GetType().Namespace)
+            }
+        ];
+    }
+}