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.

Dockerfile.builder

@@ -0,0 +1,18 @@
+# Watched Together builder image
+# Pre-built image with the .NET 9 SDK and JPRM for building and testing the plugin.
+# Build: docker build -f Dockerfile.builder -t gitea.tourolle.paris/dtourolle/watchedtogether-builder:latest .
+# Push:  docker push gitea.tourolle.paris/dtourolle/watchedtogether-builder:latest
+
+FROM mcr.microsoft.com/dotnet/sdk:9.0
+
+RUN apt-get update && apt-get install -y \
+    python3 \
+    python3-pip \
+    git \
+    jq \
+    curl \
+    && rm -rf /var/lib/apt/lists/*
+
+RUN pip install --break-system-packages jprm
+
+WORKDIR /src