Bump platform images to 0.9.0

## User request
Bump the referenced Platform Docker image versions in `agynio/bootstrap` after creating Platform release `0.9.0`.

## Specification
- Update `agyn/docker-compose.yaml` to pin:
  - `ghcr.io/agynio/platform-server:0.9.0`
  - `ghcr.io/agynio/platform-ui:0.9.0`
- No other behavior changes.
