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>

scripts/update_manifest.py

@@ -0,0 +1,113 @@
+#!/usr/bin/env python3
+"""Add a version entry to the Jellyfin plugin repository manifest.
+
+manifest.json is what Jellyfin fetches when the repository URL is added under
+Dashboard -> Plugins -> Repositories. Plugin metadata comes from build.yaml so
+there is a single source of truth.
+
+Usage:
+    scripts/update_manifest.py --version 1.0.4.0 --checksum <md5> --source-url <url>
+"""
+
+import argparse
+import datetime
+import json
+import re
+import sys
+from pathlib import Path
+
+import yaml
+
+REPO_ROOT = Path(__file__).resolve().parent.parent
+BUILD_YAML = REPO_ROOT / "build.yaml"
+MANIFEST = REPO_ROOT / "manifest.json"
+
+
+def version_key(version):
+    return tuple(int(part) for part in re.findall(r"\d+", version))
+
+
+def load_manifest():
+    if not MANIFEST.is_file():
+        return []
+    with MANIFEST.open(encoding="utf-8") as handle:
+        return json.load(handle)
+
+
+def find_plugin(manifest, guid):
+    for plugin in manifest:
+        if plugin.get("guid", "").lower() == guid.lower():
+            return plugin
+    return None
+
+
+def main():
+    parser = argparse.ArgumentParser(description=__doc__)
+    parser.add_argument("--version", required=True)
+    parser.add_argument("--checksum", required=True, help="md5 of the zip")
+    parser.add_argument("--source-url", required=True, help="download URL for the zip")
+    parser.add_argument("--changelog", default="")
+    parser.add_argument("--timestamp", default="", help="ISO 8601 UTC; defaults to now")
+    args = parser.parse_args()
+
+    with BUILD_YAML.open(encoding="utf-8") as handle:
+        cfg = yaml.safe_load(handle)
+
+    timestamp = args.timestamp or (
+        datetime.datetime.now(datetime.timezone.utc)
+        .replace(tzinfo=None)
+        .isoformat(timespec="seconds")
+        + "Z"
+    )
+
+    manifest = load_manifest()
+    plugin = find_plugin(manifest, cfg["guid"])
+    if plugin is None:
+        plugin = {"guid": cfg["guid"], "versions": []}
+        manifest.append(plugin)
+
+    # Refresh the descriptive fields so build.yaml stays authoritative.
+    plugin["name"] = cfg["name"]
+    plugin["description"] = cfg["description"].strip()
+    plugin["overview"] = cfg["overview"].strip()
+    plugin["owner"] = cfg["owner"]
+    plugin["category"] = cfg["category"]
+
+    entry = {
+        "version": args.version,
+        "changelog": (args.changelog or cfg.get("changelog", "")).strip(),
+        "targetAbi": cfg["targetAbi"],
+        "sourceUrl": args.source_url,
+        "checksum": args.checksum,
+        "timestamp": timestamp,
+    }
+
+    versions = [v for v in plugin.get("versions", []) if v.get("version") != args.version]
+    versions.append(entry)
+    versions.sort(key=lambda v: version_key(v["version"]), reverse=True)
+    plugin["versions"] = versions
+
+    # Key order matters only for readability; Jellyfin reads by name.
+    ordered = [
+        {
+            key: plugin[key]
+            for key in (
+                "guid",
+                "name",
+                "description",
+                "overview",
+                "owner",
+                "category",
+                "versions",
+            )
+            if key in plugin
+        }
+        for plugin in manifest
+    ]
+
+    MANIFEST.write_text(json.dumps(ordered, indent=4) + "\n", encoding="utf-8")
+    print(f"manifest.json now lists {args.version} at {args.source_url}")
+
+
+if __name__ == "__main__":
+    sys.exit(main())