add the ability to set a pruning policy

defaulted to --max-unused 1% in configuration.

backup.ps1

@@ -76,7 +76,7 @@ function Invoke-Maintenance {
     # prune (remove) data from the backup step. Running this separate from `forget` because
     #   `forget` only prunes when it detects removed snapshots upon invocation, not previously removed
     Write-Output "[[Maintenance]] Start pruning..." | Tee-Object -Append $SuccessLog
-    & $ResticExe prune 3>&1 2>> $ErrorLog | Tee-Object -Append $SuccessLog
+    & $ResticExe prune $SnapshotPrunePolicy 3>&1 2>> $ErrorLog | Tee-Object -Append $SuccessLog
     if(-not $?) {
         Write-Output "[[Maintenance]] Prune operation completed with errors" | Tee-Object -Append $ErrorLog | Tee-Object -Append $SuccessLog
         $maintenance_success = $false

config.ps1

@@ -13,6 +13,7 @@ $GlobalRetryAttempts = 4
 # maintenance configuration
 $SnapshotMaintenanceEnabled = $true
 $SnapshotRetentionPolicy = @("--group-by", "host", "--keep-daily", "30", "--keep-weekly", "52", "--keep-monthly", "24", "--keep-yearly", "10")
+$SnapshotPrunePolicy = @("--max-unused", "1%")
 $SnapshotMaintenanceInterval = 7
 $SnapshotMaintenanceDays = 30
 $SnapshotDeepMaintenanceDays = 90;