feat: add version update notifications to admin dashboard with configurable release channel

[Polliog]

This pull request introduces a new version update notification system for administrators, enhancing visibility into available updates and streamlining the upgrade process. It adds backend logic to check GitHub releases, exposes a new API endpoint, updates the frontend to display a notification banner in the admin dashboard, and makes the release channel configurable. Additionally, the backend /health endpoint now reports the actual package version dynamically.

Version Update Notification System:

• Added backend logic in AdminService to check for new releases on GitHub, compare versions using semver, and cache results for 6 hours. The check supports both stable and beta release channels, configurable via admin settings. [1] [2] [3] [4]
• Introduced new API endpoint GET /api/v1/admin/version-check to expose version check results to the frontend, with error handling and rate limiting.
• Updated frontend admin dashboard (+page.svelte) to fetch version check results and display a banner to admins: a blue "Update available" banner with version info and release link when an update exists, or a green "Up to date" indicator otherwise. (Fbc698f2L331R331, [1] [2] [3] [4] [5] [6] [7] [8]
• Added updates.channel setting to system settings, defaulting to stable, and made it editable from the admin settings page. [1] [2] [3]

Backend Improvements:

• /health endpoint now reports the actual version from package.json instead of a hardcoded string, ensuring version accuracy. [1] [2] [3]

Documentation:

• Updated CHANGELOG.md to document the new version update notification feature and related changes.