(ui) Surface retention warnings on run detail

Show host-level retention warnings on run detail pages so successful or
warning runs still expose scheduled prune limit issues and incomplete
snapshots that need operator attention.

src/pobsync_backend/templates/pobsync_backend/run_detail.html

@@ -179,6 +179,29 @@
     </section>
   {% endif %}
 
+  {% if retention_warning.has_warning %}
+    <section class="panel highlight warning">
+      <h2>Retention Warnings</h2>
+      <div class="stack">
+        {% if retention_warning.prune_exceeded %}
+          <div>
+            Scheduled pruning for this host would delete {{ retention_warning.delete_count }} snapshot(s), above max
+            delete {{ retention_warning.max_delete }}.
+          </div>
+        {% endif %}
+        {% if retention_warning.incomplete_count %}
+          <div>
+            {{ retention_warning.incomplete_count }} incomplete snapshot(s) exist for this host and are excluded from
+            retention cleanup.
+          </div>
+        {% endif %}
+        {% if retention_warning.error %}
+          <div>{{ retention_warning.error }}</div>
+        {% endif %}
+      </div>
+    </section>
+  {% endif %}
+
   <section class="panel">
     <h2>Raw Result</h2>
     <pre>{{ result_json }}</pre>

src/pobsync_backend/tests/test_views.py

@@ -1213,6 +1213,37 @@ class ViewTests(TestCase):
         self.assertContains(response, "Retention")
         self.assertContains(response, "Deletion blocked by --max-delete=0")
 
+    def test_run_detail_surfaces_host_retention_warnings(self) -> None:
+        self.client.force_login(self.staff_user)
+        host = HostConfig.objects.create(
+            host="web-01",
+            address="web-01.example.test",
+            retention_daily=0,
+            retention_weekly=0,
+            retention_monthly=0,
+            retention_yearly=0,
+        )
+        ScheduleConfig.objects.create(host=host, cron_expr="15 2 * * *", enabled=True, prune=True, prune_max_delete=1)
+        self._snapshot(host, "20260517-021500Z__OLDSNP1")
+        self._snapshot(host, "20260518-021500Z__OLDSNP2")
+        self._snapshot(host, "20260519-021500Z__NEWSNAP")
+        SnapshotRecord.objects.create(
+            host=host,
+            kind=SnapshotRecord.Kind.INCOMPLETE,
+            dirname="20260519-031500Z__BROKEN01",
+            path=f"/backups/{host.host}/.incomplete/20260519-031500Z__BROKEN01",
+            status="failed",
+            started_at=datetime(2026, 5, 19, 3, 15, tzinfo=timezone.utc),
+        )
+        run = BackupRun.objects.create(host=host, status=BackupRun.Status.SUCCESS, result={"ok": True})
+
+        response = self.client.get(reverse("run_detail", args=[run.id]))
+
+        self.assertEqual(response.status_code, 200)
+        self.assertContains(response, "Retention Warnings")
+        self.assertContains(response, "Scheduled pruning for this host would delete 2 snapshot(s)")
+        self.assertContains(response, "1 incomplete snapshot(s) exist for this host")
+
     def test_run_detail_infers_rsync_log_from_snapshot_path(self) -> None:
         self.client.force_login(self.staff_user)
         host = HostConfig.objects.create(host="web-01", address="web-01.example.test")

src/pobsync_backend/views.py

@@ -429,6 +429,7 @@ def run_detail(request, run_id: int):
         "failure_summary": failure.get("message") or failure.get("summary") or "",
         "prune_result": prune_result,
         "has_prune_result": bool(prune_result),
+        "retention_warning": _retention_warning_for_host(run.host, _schedule_for_host(run.host)),
         "rsync_log_path": str(rsync_log_path) if rsync_log_path is not None else "",
         "rsync_log_exists": bool(rsync_log_path and rsync_log_path.exists()),
         "rsync_log_tail": rsync_log_tail,