M5: retention policy, prune with repacking, append-only locations

- RetentionPolicy (last/daily/weekly/monthly) and PruneService: forget
  snapshots the policy drops, delete packs nothing uses and repack mostly
  unused ones by copying encrypted blobs as-is; new packs are indexed
  before old ones are removed
- a location lock keeps prune and backups apart; append-only locations
  are never pruned (cleanup via the location's lifecycle rules)
- weekly prune from the background job; occ backup:prune [--dry-run],
  backup:retention, backup:target:append-only
- tests: forget+prune frees space and the remaining snapshot restores
  identically, repack of mixed packs, dry-run preview, retention buckets

appinfo/info.xml

@@ -5,7 +5,7 @@
     <name>NG Backup</name>
     <summary>Encrypted, incremental backups of your Nextcloud with restore from the web interface</summary>
     <description><![CDATA[Work in progress, not ready for use.]]></description>
-    <version>0.0.5</version>
+    <version>0.0.6</version>
     <licence>agpl</licence>
     <author>Dillard Blom</author>
     <namespace>NgBackup</namespace>
@@ -32,6 +32,9 @@
         <command>OCA\NgBackup\Command\ListSnapshots</command>
         <command>OCA\NgBackup\Command\Browse</command>
         <command>OCA\NgBackup\Command\Restore</command>
+        <command>OCA\NgBackup\Command\Prune</command>
+        <command>OCA\NgBackup\Command\Retention</command>
+        <command>OCA\NgBackup\Command\TargetAppendOnly</command>
     </commands>
     <settings>
         <admin>OCA\NgBackup\Settings\Admin</admin>