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/IGroupService.cs

@@ -0,0 +1,41 @@
+using System;
+using System.Collections.Generic;
+using Jellyfin.Database.Implementations.Entities;
+using Jellyfin.Plugin.WatchedTogether.Configuration;
+
+namespace Jellyfin.Plugin.WatchedTogether.Services;
+
+/// <summary>
+/// Resolves shared-account groups from plugin configuration.
+/// </summary>
+public interface IGroupService
+{
+    /// <summary>
+    /// Gets the active group owning the given shared account, if any.
+    /// </summary>
+    /// <param name="sharedUserId">The shared account identifier.</param>
+    /// <returns>The group, or <c>null</c> if this user is not an enabled shared account.</returns>
+    SharedGroup? GetGroupForSharedUser(Guid sharedUserId);
+
+    /// <summary>
+    /// Gets the members of a group that are currently eligible - existing and not disabled.
+    /// </summary>
+    /// <param name="group">The group whose members to resolve.</param>
+    /// <returns>The eligible member users.</returns>
+    IReadOnlyList<User> GetEligibleMembers(SharedGroup group);
+
+    /// <summary>
+    /// Determines whether the given user is a shared account managed by this plugin, regardless
+    /// of whether its group is currently enabled.
+    /// </summary>
+    /// <param name="userId">The user identifier to test.</param>
+    /// <returns><c>true</c> if the user is a managed shared account.</returns>
+    bool IsSharedAccount(Guid userId);
+
+    /// <summary>
+    /// Removes a deleted user from every group, disabling any group left with fewer than two
+    /// members, and persists the result.
+    /// </summary>
+    /// <param name="userId">The identifier of the user that was removed.</param>
+    void PruneDeletedUser(Guid userId);
+}