Song server: central song library with on-demand downloads

- VocaluxeSongServer: ASP.NET Core server that scans an UltraStar library and
  serves the song list (ETag), single files (Range) and zip bundles; optional
  API key; Dockerfile and Gitea workflow for git.disi.dev/projects/vocaluxe-song-server
- Game: config section SongServer (Url, ApiKey). At song loading, the song text
  files are mirrored into a local cache that is loaded like a song folder;
  without connection the last library is used
- VocaluxeLib: CSongFiles hook for all song file access; media are downloaded on
  demand (covers in the background, preview audio, everything before singing
  with progress in the sing screen)
- Tests for scanner, API and downloads (incl. resume); AGENTS.md for the fork

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.gitea/workflows/song-server.yml

@@ -0,0 +1,47 @@
+# Builds the song server image and pushes it to the Gitea registry:
+#   git.disi.dev/projects/vocaluxe-song-server:<version> and :latest
+# Release: set VocaluxeSongServer/VERSION, then push the tag songserver-v<version> (e.g. songserver-v0.1.0).
+# Manual runs (workflow_dispatch) push :dev only.
+name: Song server image
+
+env:
+  DOCKER_API_VERSION: '1.43'
+  IMAGE: git.disi.dev/projects/vocaluxe-song-server
+
+on:
+  push:
+    tags:
+      - 'songserver-v*'
+  workflow_dispatch:
+
+jobs:
+  image:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v5
+      - name: Determine tags
+        id: tags
+        run: |
+          VERSION=$(tr -d '[:space:]' < VocaluxeSongServer/VERSION)
+          if [ "${GITHUB_REF_TYPE}" = "tag" ]; then
+            if [ "${GITHUB_REF_NAME}" != "songserver-v${VERSION}" ]; then
+              echo "Tag ${GITHUB_REF_NAME} does not match VocaluxeSongServer/VERSION (${VERSION})"
+              exit 1
+            fi
+            echo "tags=${VERSION} latest" >> "$GITHUB_OUTPUT"
+          else
+            echo "tags=dev" >> "$GITHUB_OUTPUT"
+          fi
+      - name: Login to Gitea container registry
+        uses: docker/login-action@v3
+        with:
+          username: gitearobot
+          password: ${{ secrets.PACKAGE_GITEA_PAT }}
+          registry: git.disi.dev
+      - name: Build and publish
+        run: |
+          docker build -t "${IMAGE}:build" -f VocaluxeSongServer/Dockerfile .
+          for TAG in ${{ steps.tags.outputs.tags }}; do
+            docker tag "${IMAGE}:build" "${IMAGE}:${TAG}"
+            docker push "${IMAGE}:${TAG}"
+          done