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/IProvisioningService.cs

@@ -0,0 +1,50 @@
+using System;
+using System.Collections.Generic;
+using System.Threading.Tasks;
+using Jellyfin.Plugin.WatchedTogether.Configuration;
+
+namespace Jellyfin.Plugin.WatchedTogether.Services;
+
+/// <summary>
+/// Creates, updates and removes shared accounts and their groups.
+/// </summary>
+public interface IProvisioningService
+{
+    /// <summary>
+    /// Creates a shared account for the given members and records the group.
+    /// </summary>
+    /// <param name="memberIds">The members whose passwords will unlock the account. At least two.</param>
+    /// <param name="name">An explicit account name, or <c>null</c> to generate one from the member names.</param>
+    /// <param name="enableAllFolders">Whether the shared account may access all libraries.</param>
+    /// <param name="enabledFolders">Explicit library identifiers, used when <paramref name="enableAllFolders"/> is false.</param>
+    /// <returns>The created group.</returns>
+    Task<SharedGroup> CreateGroupAsync(
+        IReadOnlyList<Guid> memberIds,
+        string? name,
+        bool enableAllFolders,
+        IReadOnlyList<Guid>? enabledFolders);
+
+    /// <summary>
+    /// Replaces the membership and options of an existing group.
+    /// </summary>
+    /// <param name="sharedUserId">The shared account identifying the group.</param>
+    /// <param name="memberIds">The new member list. At least two.</param>
+    /// <param name="syncUnwatched">Whether unwatched state propagates too.</param>
+    /// <param name="syncPlayCount">Whether play counts are raised on watch.</param>
+    /// <param name="isDisabled">Whether the group is suspended.</param>
+    /// <returns>The updated group.</returns>
+    Task<SharedGroup> UpdateGroupAsync(
+        Guid sharedUserId,
+        IReadOnlyList<Guid> memberIds,
+        bool syncUnwatched,
+        bool syncPlayCount,
+        bool isDisabled);
+
+    /// <summary>
+    /// Removes a group, optionally deleting its shared account.
+    /// </summary>
+    /// <param name="sharedUserId">The shared account identifying the group.</param>
+    /// <param name="deleteSharedUser">Whether to delete the shared Jellyfin account as well.</param>
+    /// <returns>A task representing the removal.</returns>
+    Task DeleteGroupAsync(Guid sharedUserId, bool deleteSharedUser);
+}