Add a simple version check to inform users of outdated versions

[lukehinds]

Description:

Currently, there’s no built-in way for users to know if their instance of CodeGate is running an outdated version. This can cause a sub-par experience or potential security or compatibility issues.

I’d like to propose a simple version-checking system that:

1. Stores the current local version (e.g., in a VERSION file at the repo root or an environment variable).
2. Fetches the latest version from GitHub (via either:
   • Releases API, or
   • The raw VERSION file on the main branch:
     ``` https://raw.githubusercontent.com///main/VERSION ``` ).
3. Compares the local version to the latest version.
4. Notifies users (in the frontend or logs) when a newer version is available.

Why this is needed

• No current version-awareness: At present, there’s no way of knowing if a user is running an outdated version of CodeGate.
• User-friendly: A quick version check can warn users when they need to update CodeGate.
• Lightweight: Fetching a single file or release tag from GitHub is straightforward and doesn’t require infrastructure.

Proposed Steps

1. Create/maintain a VERSION file at the root of the repository or using the existing https://github.com/stacklok/codegate/blob/main/src/codegate/__init__.py#L17, and have it updated each time we release new code (automate via GH actions when we publish a release).
2. Implement a /api/version endpoint in the FastAPI backend that returns the local (running) version.
3. Add a check against GitHub (e.g., /api/update-status), fetching the latest release or raw VERSION file from the main branch.
4. Compare local vs. remote versions and return a response indicating whether the app is up to date (e.g., "is_latest": true/false).
5. Notify the front end (React) whenever a mismatch is found, so users can see a banner or message like, “A new version of CodeGate is available!”

[alex-mcgovern]

In implementing this (polling the server from the UI, which checks Github for latest release) I got myself rate limited by the Github API pretty quickly. Going to rethink the polling mechanism in the UI while I work on this.