VM Backup usage billed at last-reported size for the entire billing period

### problem

When `EVENT_VM_BACKUP_USAGE_METRIC` events are processed, `Usage Manager` performs an in-place UPDATE of the single usage_backup row for a given (vm_id, backup_offering_id) pair. This means only the most recently received size is retained. When BackupUsageParser runs at billing time, it reads that single row and applies its size to the entire billing window duration, regardless of when the size actually changed.

### versions

ACS 4.20

### The steps to reproduce the bug

The problem is independent of the provider but I am using NAS for testing:
1. Set usage.stats.job.exec.time to `10:00`
2. Assign a backup offering to a VM at `11:00`
3. Create a backup of size 1GB at `11:00`
4. Create a backup of size 99GB at `09:00` the next day
5. Wait for usage job to run and finish at `10:00`

### What to do about it?

**Expected Behavior**
The user should be charged proportionally: ~23 hours at 1 GB + ~1 hours at 100 GB

**Actual Behavior**
The user is charged at 100 GB for the full 24 hours because the last metric event overwrote the size column in-place, erasing all history.

**Fix would be to:**

- Find the active row in `usage_backup` (removed IS NULL) for the (vm_id, backup_offering_id).
- Set its removed = eventDate.
- persist a new UsageBackupVO with the updated size/protectedSize and created = eventDate.
- This matches the existing pattern in UsageNetworksDaoImpl and UsageVpcDaoImpl.

Start with Usage Manager processing of EVENT_VM_BACKUP_USAGE_METRIC and trace how usage_backup rows are handled by BackupUsageParser. Compare the existing patterns in UsageNetworksDaoImpl and UsageVpcDaoImpl, then verify that size changes create historical intervals so billing charges each size for only the time it was active.

Written by the indexing model from the issue text.