(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/backup_runner.py

@@ -111,7 +111,7 @@ def execute_backup_run(
             )
         except Exception as exc:
             result["prune"] = {"ok": False, "error": str(exc), "type": type(exc).__name__}
-            run.status = BackupRun.Status.FAILED
+            run.status = BackupRun.Status.WARNING
             run.result = result
             run.snapshot = snapshot_record
             run.save(
@@ -125,7 +125,6 @@ def execute_backup_run(
                     "result",
                 ],
             )
-            raise
 
     run.snapshot = snapshot_record
     run.result = result