Add repository metadata synchronization

### Description
Enrich listings automatically from their source repositories.

### Potential metadata:
- stars
- license
- latest release/version
- last update
- archived status
- repository description

### Acceptance criteria
- Repository metadata is cached rather than fetched on every request
- Failed GitHub requests don't break project pages
- Metadata has `last_synced_at`
- Rate limits are handled

This could later make stale/abandoned bot listings obvious.

Start by locating the listing/project-page data flow and the existing GitHub request entry point. Define where repository metadata is stored and read, then verify the acceptance criteria: cached data with last_synced_at, graceful failed requests, and rate-limit handling without breaking project pages.

Written by the indexing model from the issue text.