(feature) Capture structured backup statistics

Parse rsync --stats output into structured run metrics for file counts,
transferred bytes, literal data, matched data, speedup, and estimated
link-dest savings.

Store collected stats on backup run results and successful snapshot metadata,
including snapshot data usage and backup-root capacity details for future
dashboard graphs and disk-full projections.

Render the collected metrics on run and snapshot detail pages, with tests
covering parsing, metadata persistence, and UI output.

src/pobsync_backend/tests/test_views.py

@@ -753,6 +753,18 @@ class ViewTests(TestCase):
                     "prune_max_delete": 10,
                     "prune_protect_bases": False,
                 },
+                "stats": {
+                    "duration_seconds": 12,
+                    "rsync": {
+                        "files_total": 10,
+                        "files_transferred": 2,
+                        "total_file_size_bytes": 2000,
+                        "total_transferred_file_size_bytes": 500,
+                        "literal_data_bytes": 500,
+                        "matched_data_bytes": 1500,
+                        "link_dest_estimated_savings_bytes": 1500,
+                    },
+                },
             },
         )
 
@@ -766,6 +778,9 @@ class ViewTests(TestCase):
         self.assertContains(response, "Requested Options")
         self.assertContains(response, "Dry run:</strong> yes")
         self.assertContains(response, "Verbose rsync output:</strong> yes")
+        self.assertContains(response, "Stats")
+        self.assertContains(response, "Files seen:</strong> 10")
+        self.assertContains(response, "Estimated link-dest saving")
         self.assertContains(response, "&quot;ok&quot;: true")
         self.assertContains(response, reverse("snapshot_detail", args=[snapshot.id]))
 
@@ -811,7 +826,30 @@ class ViewTests(TestCase):
         self.client.force_login(self.staff_user)
         host = HostConfig.objects.create(host="web-01", address="web-01.example.test")
         base = self._snapshot(host, "20260518-021500Z__BASESNAP")
-        base.metadata = {"status": "success", "snapshot_id": "BASESNAP"}
+        base.metadata = {
+            "status": "success",
+            "snapshot_id": "BASESNAP",
+            "stats": {
+                "duration_seconds": 20,
+                "rsync": {
+                    "files_total": 100,
+                    "files_transferred": 4,
+                    "total_file_size_bytes": 10_000,
+                    "link_dest_estimated_savings_bytes": 7_000,
+                },
+                "storage": {
+                    "snapshot": {
+                        "apparent_size_bytes": 10_000,
+                        "allocated_size_bytes": 3_000,
+                        "hardlinked_files": 9,
+                    },
+                    "capacity": {
+                        "used_percent": 30.5,
+                        "available_bytes": 1_000_000,
+                    },
+                },
+            },
+        }
         base.save(update_fields=["metadata"])
         child = self._snapshot(host, "20260519-021500Z__CHILDSNP")
         child.base = base
@@ -824,6 +862,9 @@ class ViewTests(TestCase):
         self.assertEqual(response.status_code, 200)
         self.assertContains(response, base.dirname)
         self.assertContains(response, "BASESNAP")
+        self.assertContains(response, "Stats")
+        self.assertContains(response, "Files seen:</strong> 100")
+        self.assertContains(response, "Hardlinked files:</strong> 9")
         self.assertContains(response, child.dirname)
         self.assertContains(response, f"Run {run.id}")
         self.assertContains(response, reverse("run_detail", args=[run.id]))