(bugfix) Mark retention failures as backup warnings

Add a warning status for BackupRun records so successful snapshots are not
reported as failed when post-run SQL retention fails.

Keep the prune error in the run result, link the successful snapshot, and
let the management command complete with a warning instead of raising a
backup failure.

Include warning runs in backup trend summaries and add a regression test
for successful backups with failed retention cleanup.

src/pobsync_backend/management/commands/run_pobsync_backup.py

@@ -60,5 +60,8 @@ class Command(BaseCommand):
         if run.status == BackupRun.Status.SUCCESS:
             self.stdout.write(self.style.SUCCESS(f"Backup completed for {host.host}."))
             return
+        if run.status == BackupRun.Status.WARNING:
+            self.stdout.write(self.style.WARNING(f"Backup completed with warnings for {host.host}; run id={run.id}"))
+            return
 
         raise CommandError(f"Backup failed for {host.host}; run id={run.id}")