(feature) Link rsync logs from backup run detail

Record the final rsync log path for successful real backup runs, matching
the existing dry-run and failure result payloads.

Add a staff-only run log endpoint and surface the link on run detail pages,
including fallback log discovery for older runs based on snapshot_path.

Cover direct log links and inferred scheduled backup logs with view tests.

src/pobsync_backend/tests/test_run_scheduled_config_source.py

@@ -242,6 +242,7 @@ class RunScheduledConfigSourceTests(SimpleTestCase):
             meta_text = meta_path.read_text(encoding="utf-8")
 
         self.assertTrue(result["ok"])
+        self.assertEqual(result["log"], str(Path(result["snapshot"]) / "meta" / "rsync.log"))
         self.assertEqual(result["stats"]["rsync"]["files_total"], 10)
         self.assertEqual(result["stats"]["rsync"]["files_transferred"], 2)
         self.assertEqual(result["stats"]["rsync"]["link_dest_estimated_savings_bytes"], 1500)