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/models.py

@@ -82,6 +82,13 @@ class BackupRun(models.Model):
     started_at = models.DateTimeField(null=True, blank=True)
     ended_at = models.DateTimeField(null=True, blank=True)
     snapshot_path = models.CharField(max_length=1024, blank=True)
+    snapshot = models.ForeignKey(
+        "SnapshotRecord",
+        on_delete=models.SET_NULL,
+        null=True,
+        blank=True,
+        related_name="backup_runs",
+    )
     base_path = models.CharField(max_length=1024, blank=True)
     rsync_exit_code = models.IntegerField(null=True, blank=True)
     result = models.JSONField(default=dict, blank=True)