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/Services/IDynamicGroupService.cs

@@ -0,0 +1,29 @@
+using System.Threading.Tasks;
+using Jellyfin.Plugin.WatchedTogether.Configuration;
+
+namespace Jellyfin.Plugin.WatchedTogether.Services;
+
+/// <summary>
+/// Creates shared accounts on demand from a separator-joined username typed at the login screen.
+/// </summary>
+public interface IDynamicGroupService
+{
+    /// <summary>
+    /// Attempts to authenticate a not-yet-existing shared account named like "alice+bob", creating
+    /// it if the submitted password belongs to one of the named members.
+    /// </summary>
+    /// <param name="enteredUsername">The username typed at the login screen.</param>
+    /// <param name="password">The submitted password.</param>
+    /// <returns>
+    /// The created group and the shared account's username, or <c>null</c> if the name is not a
+    /// valid member combination or no named member's password matched.
+    /// </returns>
+    Task<DynamicGroupResult?> TryCreateFromLoginAsync(string enteredUsername, string password);
+}
+
+/// <summary>
+/// The outcome of a successful on-demand group creation.
+/// </summary>
+/// <param name="Group">The group that was created.</param>
+/// <param name="SharedUsername">The username of the shared account.</param>
+public record DynamicGroupResult(SharedGroup Group, string SharedUsername);