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.

.gitea/workflows/build.yaml

@@ -0,0 +1,89 @@
+name: '🏗️ Build Plugin'
+
+on:
+  push:
+    branches:
+      - master
+    paths-ignore:
+      - '**/*.md'
+  pull_request:
+    branches:
+      - master
+    paths-ignore:
+      - '**/*.md'
+  workflow_dispatch:
+
+jobs:
+  build:
+    runs-on: linux/amd64
+    container:
+      image: gitea.tourolle.paris/dtourolle/watchedtogether-builder:latest
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+        with:
+          path: build-${{ github.run_id }}
+
+      - name: Cache NuGet packages
+        uses: actions/cache@v3
+        with:
+          path: ~/.nuget/packages
+          key: nuget-${{ hashFiles('**/Jellyfin.Plugin.WatchedTogether.csproj', '**/Jellyfin.Plugin.WatchedTogether.Tests.csproj') }}
+          restore-keys: nuget-
+
+      - name: Restore dependencies
+        working-directory: build-${{ github.run_id }}
+        run: dotnet restore Jellyfin.Plugin.WatchedTogether.sln
+
+      - name: Compute build version
+        id: version
+        run: |
+          # Date-based version so a side-loaded build is identifiable in Jellyfin.
+          DATE=$(date -u +"%Y%m%d")
+          VERSION="1.0.${DATE}.${{ github.run_number }}"
+          if [ -n "${{ github.event.pull_request.number }}" ]; then
+            LABEL="pr${{ github.event.pull_request.number }}"
+          else
+            LABEL="master"
+          fi
+          echo "version=${VERSION}" >> $GITHUB_OUTPUT
+          echo "label=${LABEL}" >> $GITHUB_OUTPUT
+          echo "Build version: ${VERSION} (${LABEL})"
+
+      - name: Set build version
+        working-directory: build-${{ github.run_id }}
+        run: |
+          sed -i "s/^version:.*/version: \"${{ steps.version.outputs.version }}\"/" build.yaml
+
+      - name: Build solution
+        working-directory: build-${{ github.run_id }}
+        run: dotnet build Jellyfin.Plugin.WatchedTogether.sln --configuration Release --no-restore --no-self-contained /m:1
+
+      - name: Run tests
+        working-directory: build-${{ github.run_id }}
+        run: dotnet test Jellyfin.Plugin.WatchedTogether.sln --no-build --configuration Release --verbosity normal
+
+      - name: Build Jellyfin Plugin
+        id: jprm
+        working-directory: build-${{ github.run_id }}
+        run: |
+          mkdir -p artifacts
+          jprm --verbosity=debug plugin build .
+          ARTIFACT=$(find . -name "*.zip" -type f -print -quit | sed 's|^\./||')
+          LATEST="artifacts/watchedtogether_latest.zip"
+          cp "${ARTIFACT}" "${LATEST}"
+          echo "artifact=${LATEST}" >> $GITHUB_OUTPUT
+          echo "Found artifact: ${ARTIFACT} -> ${LATEST}"
+
+      - name: Upload build artifact
+        uses: actions/upload-artifact@v3
+        with:
+          name: watchedtogether-${{ steps.version.outputs.label }}-${{ steps.version.outputs.version }}
+          path: build-${{ github.run_id }}/${{ steps.jprm.outputs.artifact }}
+          retention-days: 30
+          if-no-files-found: error
+
+      - name: Cleanup
+        if: always()
+        run: rm -rf build-${{ github.run_id }}