catalog: document minimum Emby version requirement

[CCoupel]

Context

Emby catalog listings include a minimum server version field. This must be explicitly defined and documented.

Problem

The minimum Emby version required by VirtualLib is not documented anywhere (README, csproj, catalog manifest). The plugin uses APIs that may not be available on older Emby versions.

Fix

1. Determine the minimum Emby version by checking the oldest API used:

   • `IScheduledTask` interface version
   • `BaseApiController` availability
   • `[Route]` attribute routing support

2. Document in `README.md`:
   ```
   Requires: Emby Server 4.8.x or later
   ```

3. Set `targetAbi` in the catalog manifest JSON accordingly

4. Optionally add to `VirtualLib.csproj`:
   ```xml
   
   ```

Priority

Low — Required for catalog submission metadata but does not affect functionality.