feat: move restic (backrest) to core stack

- Add backrest service to core/compose.yaml with CORE_RESTIC_ prefix
- Update arty.yml with CORE_RESTIC_* environment variables
- Remove restic from compose.yaml includes
- Backrest now part of core infrastructure stack

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

compose.yaml

@@ -4,7 +4,6 @@ include:
   - sexy/compose.yaml
   - util/compose.yaml
   - ai/compose.yaml
-  - restic/compose.yaml
   - net/compose.yaml
   - media/compose.yaml
   - dev/compose.yaml