fix: make the release workflow build and package a real Jellyfin plugin

The release workflow was copied from another project and could not succeed on
any tag: it installed the .NET 8 SDK for a net9.0 project, published a
JellyfinSso.csproj that does not exist, called a missing update_manifest.py,
and zipped the entire publish tree instead of the three DLLs declared in
build.yaml.

Replace it with a tag-driven release:

- Build with the .NET 9 SDK, injecting the tag version via -p:Version,
  -p:AssemblyVersion and -p:FileVersion so the assembly no longer carries a
  hardcoded 1.0.3.0.
- Package via scripts/build_plugin.py, which emits the JPRM layout Jellyfin
  expects: the build.yaml artifacts plus a generated meta.json. The previous
  zips shipped without meta.json, which only worked for repository installs
  because Jellyfin synthesises one from the folder name.
- Update manifest.json via scripts/update_manifest.py, keeping build.yaml as
  the single source of truth for plugin metadata.
- Commit the zip and manifest to main and attach the zip to the Gitea release.
  Pushing uses the injected token, falling back to a RELEASE_TOKEN secret when
  the built-in one lacks write access.

Also fix the published download URLs. manifest.json and the README pointed at
pascallinxweiler/jellyfinplugin, which 404s; the repository is jellyfinsso, so
the plugin repository could not be added to Jellyfin and no version could be
downloaded. The workflow now derives the URL from GITHUB_REPOSITORY.

Add a build workflow so main and pull requests are compiled and packaged.

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

.gitea/workflows/build.yml

@@ -0,0 +1,42 @@
+name: Build Plugin
+
+on:
+  push:
+    branches:
+      - main
+    paths-ignore:
+      - '**/*.md'
+  pull_request:
+    paths-ignore:
+      - '**/*.md'
+  workflow_dispatch:
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Setup .NET
+        uses: actions/setup-dotnet@v4
+        with:
+          dotnet-version: '9.0.x'
+
+      - name: Setup Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: '3.x'
+
+      - name: Install packaging dependencies
+        run: python -m pip install --quiet --upgrade pyyaml
+
+      - name: Build and package plugin
+        id: package
+        run: python scripts/build_plugin.py --output "$RUNNER_TEMP/dist"
+
+      - name: Upload artifact
+        uses: actions/upload-artifact@v3
+        with:
+          name: ${{ steps.package.outputs.zip_name }}
+          path: ${{ runner.temp }}/dist/${{ steps.package.outputs.zip_name }}