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/tests/test_run_backup_records_snapshot.py

@@ -43,8 +43,10 @@ class RunBackupRecordsSnapshotTests(TestCase):
                 call_command("run_pobsync_backup", host.host, prefix=str(Path(tmp) / "home"), stdout=StringIO())
 
         self.assertEqual(BackupRun.objects.count(), 1)
+        run = BackupRun.objects.get()
         self.assertEqual(SnapshotRecord.objects.count(), 1)
         record = SnapshotRecord.objects.get()
+        self.assertEqual(run.snapshot, record)
         self.assertEqual(record.host, host)
         self.assertEqual(record.kind, "scheduled")
         self.assertEqual(record.status, "success")
@@ -74,6 +76,7 @@ class RunBackupRecordsSnapshotTests(TestCase):
         run = BackupRun.objects.get()
         self.assertEqual(run.status, BackupRun.Status.FAILED)
         record = SnapshotRecord.objects.get()
+        self.assertEqual(run.snapshot, record)
         self.assertEqual(record.kind, "incomplete")
         self.assertEqual(record.status, "failed")
 
@@ -99,4 +102,5 @@ class RunBackupRecordsSnapshotTests(TestCase):
                 )
 
         self.assertEqual(BackupRun.objects.count(), 1)
+        self.assertIsNone(BackupRun.objects.get().snapshot)
         self.assertEqual(SnapshotRecord.objects.count(), 0)