(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/migrations/0008_alter_backuprun_status.py

@@ -0,0 +1,27 @@
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("pobsync_backend", "0007_filesystem_ssh_credentials"),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name="backuprun",
+            name="status",
+            field=models.CharField(
+                choices=[
+                    ("queued", "Queued"),
+                    ("running", "Running"),
+                    ("success", "Success"),
+                    ("warning", "Warning"),
+                    ("failed", "Failed"),
+                    ("cancelled", "Cancelled"),
+                ],
+                default="queued",
+                max_length=16,
+            ),
+        ),
+    ]