feat: link backup runs to snapshot records

Add a nullable SnapshotRecord foreign key to BackupRun and populate it
when run_pobsync_backup records a completed or failed snapshot. Keep the
existing snapshot_path for audit compatibility while making run-to-snapshot
navigation explicit in the database and admin.

src/pobsync_backend/migrations/0004_backuprun_snapshot.py

@@ -0,0 +1,24 @@
+from __future__ import annotations
+
+import django.db.models.deletion
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("pobsync_backend", "0003_structured_config_fields"),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name="backuprun",
+            name="snapshot",
+            field=models.ForeignKey(
+                blank=True,
+                null=True,
+                on_delete=django.db.models.deletion.SET_NULL,
+                related_name="backup_runs",
+                to="pobsync_backend.snapshotrecord",
+            ),
+        ),
+    ]