(feature) Add backup trend forecasting to the Django UI

Extend derived backup statistics with average daily new data and an
estimated days-until-full forecast based on recent successful real runs.

Show the new forecast metrics on the dashboard and add compact per-run
trend bars on the host detail page so new data and matched link-dest data
are easier to compare at a glance.

Keep the implementation migration-free by deriving everything from the
existing BackupRun result stats payload.

src/pobsync_backend/tests/test_views.py

@@ -85,6 +85,8 @@ class ViewTests(TestCase):
         self.assertEqual(response.status_code, 200)
         self.assertContains(response, "Backup Root Used")
         self.assertContains(response, "Runs Until Full")
+        self.assertContains(response, "Avg Daily New")
+        self.assertContains(response, "Days Until Full")
         self.assertContains(response, "10")
         self.assertContains(response, f"Run {run.id}")
         self.assertContains(response, "1000")
@@ -582,16 +584,38 @@ class ViewTests(TestCase):
                 },
             },
         )
+        BackupRun.objects.create(
+            host=host,
+            status=BackupRun.Status.SUCCESS,
+            snapshot=snapshot,
+            started_at=datetime(2026, 5, 18, 2, 15, tzinfo=timezone.utc),
+            result={
+                "ok": True,
+                "dry_run": False,
+                "stats": {
+                    "duration_seconds": 35,
+                    "rsync": {
+                        "files_total": 150,
+                        "literal_data_bytes": 1024,
+                        "matched_data_bytes": 4096,
+                    },
+                },
+            },
+        )
 
         response = self.client.get(reverse("host_detail", args=[host.host]))
 
         self.assertEqual(response.status_code, 200)
         self.assertContains(response, "Backup Trends")
         self.assertContains(response, "Avg New Data")
+        self.assertContains(response, "Avg Daily New")
         self.assertContains(response, "45s")
         self.assertContains(response, "250")
         self.assertContains(response, "2.0")
         self.assertContains(response, "KB")
+        self.assertContains(response, "Run data trend")
+        self.assertContains(response, "width: 100%")
+        self.assertContains(response, "width: 50%")
 
     def test_prepare_host_directories_action_creates_missing_directories(self) -> None:
         self.client.force_login(self.staff_user)