(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/models.py

@@ -105,6 +105,7 @@ class BackupRun(models.Model):
         QUEUED = "queued", "Queued"
         RUNNING = "running", "Running"
         SUCCESS = "success", "Success"
+        WARNING = "warning", "Warning"
         FAILED = "failed", "Failed"
         CANCELLED = "cancelled", "Cancelled"