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/Configuration/SharedGroup.cs

@@ -0,0 +1,49 @@
+using System;
+using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
+
+namespace Jellyfin.Plugin.WatchedTogether.Configuration;
+
+/// <summary>
+/// The association between one shared account and the members who may unlock it.
+/// </summary>
+/// <remarks>
+/// Membership is stored as GUIDs rather than being parsed out of the shared account's username.
+/// The default separator ('+') is itself a legal username character, so a name like "alice+bob"
+/// is ambiguous between the group [alice, bob] and a single user literally called "alice+bob".
+/// GUIDs remove that ambiguity and support any number of members.
+/// </remarks>
+public class SharedGroup
+{
+    /// <summary>
+    /// Gets or sets the identifier of the shared account that members log into collectively.
+    /// </summary>
+    public Guid SharedUserId { get; set; }
+
+    /// <summary>
+    /// Gets or sets the identifiers of the members whose passwords unlock the shared account,
+    /// and whose own accounts receive its watched state. A usable group has at least two.
+    /// </summary>
+    [SuppressMessage("Usage", "CA2227:Collection properties should be read only", Justification = "Plugin configuration is round-tripped by the XML serializer, which requires a settable List<T>.")]
+    [SuppressMessage("Design", "CA1002:Do not expose generic lists", Justification = "Plugin configuration is round-tripped by the XML serializer, which requires a settable List<T>.")]
+    public List<Guid> MemberUserIds { get; set; } = new();
+
+    /// <summary>
+    /// Gets or sets a value indicating whether marking something unwatched on the shared account
+    /// also marks it unwatched for every member. When false, only the transition to watched
+    /// propagates.
+    /// </summary>
+    public bool SyncUnwatched { get; set; } = true;
+
+    /// <summary>
+    /// Gets or sets a value indicating whether a member's play count is raised to at least one
+    /// when an item becomes watched. Play counts are never decremented.
+    /// </summary>
+    public bool SyncPlayCount { get; set; }
+
+    /// <summary>
+    /// Gets or sets a value indicating whether this group is suspended. A group drops out of both
+    /// authentication and sync while disabled - set automatically if it falls below two members.
+    /// </summary>
+    public bool IsDisabled { get; set; }
+}