Previously, SyncGuestBackupTimes matched backups to guests using only VMID.
This caused newly created containers to incorrectly show old backup times
from different containers on other Proxmox instances that happened to have
the same VMID.
Now uses composite key (instance+VMID) for PVE storage backups to ensure
proper isolation. PBS backups still use VMID matching (since they aggregate
from multiple sources) but only as a fallback.
Fixes issue where ollama LXC showed 'last backup 3 months ago' despite
being created yesterday.
Add tests for the remaining uncovered State methods:
- SyncGuestBackupTimes: Tests backup time sync from storage and PBS backups
- UpdateStorageBackupsForInstance: Tests storage backup updates with instance isolation
Improves internal/models coverage from 89.6% to 95.6%.
