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.

.github/workflows/build.yaml

@@ -1,18 +0,0 @@
-name: '🏗️ Build Plugin'
-
-on:
-  push:
-    branches:
-      - master
-    paths-ignore:
-      - '**/*.md'
-  pull_request:
-    branches:
-      - master
-    paths-ignore:
-      - '**/*.md'
-  workflow_dispatch:
-
-jobs:
-  call:
-    uses: jellyfin/jellyfin-meta-plugins/.github/workflows/build.yaml@master

.github/workflows/changelog.yaml

@@ -1,20 +0,0 @@
-name: '📝 Create/Update Release Draft & Release Bump PR'
-
-on:
-  push:
-    branches:
-      - master
-    paths-ignore:
-      - build.yaml
-  workflow_dispatch:
-  repository_dispatch:
-    types:
-      - update-prep-command
-
-jobs:
-  call:
-    uses: jellyfin/jellyfin-meta-plugins/.github/workflows/changelog.yaml@master
-    with:
-      repository-name: jellyfin/jellyfin-plugin-template
-    secrets:
-      token: ${{ secrets.GITHUB_TOKEN }}

.github/workflows/command-dispatch.yaml

@@ -1,13 +0,0 @@
-# Allows for the definition of PR and Issue /commands
-name: '📟 Slash Command Dispatcher'
-
-on:
-  issue_comment:
-    types:
-      - created
-
-jobs:
-  call:
-    uses: jellyfin/jellyfin-meta-plugins/.github/workflows/command-dispatch.yaml@master
-    secrets:
-      token: .

.github/workflows/command-rebase.yaml

@@ -1,16 +0,0 @@
-name: '🔀 PR Rebase Command'
-
-on:
-  repository_dispatch:
-    types:
-      - rebase-command
-
-jobs:
-  call:
-    uses: jellyfin/jellyfin-meta-plugins/.github/workflows/command-rebase.yaml@master
-    with:
-      rebase-head: ${{ github.event.client_payload.pull_request.head.label }}
-      repository-full-name: ${{ github.event.client_payload.github.payload.repository.full_name }}
-      comment-id: ${{ github.event.client_payload.github.payload.comment.id }}
-    secrets:
-      token: ${{ secrets.GITHUB_TOKEN }}

.github/workflows/publish.yaml

@@ -1,18 +0,0 @@
-name: '🚀 Publish Plugin'
-
-on:
-  release:
-    types:
-      - released
-  workflow_dispatch:
-
-jobs:
-  call:
-    uses: jellyfin/jellyfin-meta-plugins/.github/workflows/publish.yaml@master
-    with:
-      version: ${{ github.event.release.tag_name }}
-      is-unstable: ${{ github.event.release.prerelease }}
-    secrets:
-      deploy-host: ${{ secrets.DEPLOY_HOST }}
-      deploy-user: ${{ secrets.DEPLOY_USER }}
-      deploy-key: ${{ secrets.DEPLOY_KEY }}

.github/workflows/scan-codeql.yaml

@@ -1,20 +0,0 @@
-name: '🔬 Run CodeQL'
-
-on:
-  push:
-    branches: [ master ]
-    paths-ignore:
-      - '**/*.md'
-  pull_request:
-    branches: [ master ]
-    paths-ignore:
-      - '**/*.md'
-  schedule:
-    - cron: '24 2 * * 4'
-  workflow_dispatch:
-
-jobs:
-  call:
-    uses: jellyfin/jellyfin-meta-plugins/.github/workflows/scan-codeql.yaml@master
-    with:
-      repository-name: jellyfin/jellyfin-plugin-template

.github/workflows/sync-labels.yaml

@@ -1,12 +0,0 @@
-name: '🏷️ Sync labels'
-
-on:
-  schedule:
-    - cron: '0 0 1 * *'
-  workflow_dispatch:
-
-jobs:
-  call:
-    uses: jellyfin/jellyfin-meta-plugins/.github/workflows/sync-labels.yaml@master
-    secrets:
-      token: ${{ secrets.GITHUB_TOKEN }}

.github/workflows/test.yaml

@@ -1,18 +0,0 @@
-name: '🧪 Test Plugin'
-
-on:
-  push:
-    branches:
-      - master
-    paths-ignore:
-      - '**/*.md'
-  pull_request:
-    branches:
-      - master
-    paths-ignore:
-      - '**/*.md'
-  workflow_dispatch:
-
-jobs:
-  call:
-    uses: jellyfin/jellyfin-meta-plugins/.github/workflows/test.yaml@master