feat(synology): add Synology DSM 7 SPK package builder and deployment support

[erseco]

Summary

This PR adds official packaging support for Synology NAS DiskStation Manager (DSM 7.0 / 7.1 / 7.2+) via an official lightweight SPK package (packaging/synology/).

The SPK acts as a native wrapper that integrates eXeLearning into Synology DSM, orchestrating the official multi-architecture Docker image via Synology's Container Manager (docker-project resource).

Key Features & Changes

1. Lightweight SPK Builder (packaging/synology/build-spk.sh):

   • Builds a clean POSIX ustar .spk package archive.
   • Pinned versioning linking the package to the matching official Docker image tag.
   • Unit tests covering the build script and structure in packaging/synology/build-spk.spec.ts.

2. DSM Desktop & Package Center Integration:

   • Adds ui/config and INFO metadata (dsmuidir="ui", dsmappname="SYNO.SDS.eXeLearning", adminport="8085").
   • Enables native DSM desktop launcher and the Package Center "Open" button to access the web UI directly.
   • Package icons (64x64 and 256x256) for DSM menu.

3. Storage & Port Configuration:

   • Uses standard Docker named volume (exelearning_data:/mnt/data) for automatic directory and permission provisioning, ensuring full data persistence across container updates and volume neutrality (volume1, volume2, etc.).
   • Default external port mapped to 8085:8080 to avoid conflicts on common Synology NAS services.

4. Automation & Documentation:

   • Added make package-synology VERSION=x.y.z target in Makefile.
   • Added GitHub Actions workflow in .github/workflows/synology-release.yml.
   • Comprehensive deployment and SSO federation guide in doc/deploy/synology.md and integrated into mkdocs.yml.

Verification

• make fix passed cleanly.
• bun test ./packaging/synology/build-spk.spec.ts passed (100% assertions green).
• Tested and verified on physical Synology NAS (DSM 7.2.2 on DS918+).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ All tests successful. No failed tests found.

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[codecov]

Bundle Report

Bundle size has no change ✅