(feature) Add read-only retention plan view

Add a staff-only retention plan page for each host using the SQL-backed
retention service. Link it from the host detail page and show policy settings,
keep reasons, and snapshots that would be deleted for scheduled, manual, or all
snapshot kinds.

Keep the flow non-destructive for now, validate query parameters, and cover the
view with tests for rendering, base protection, and invalid kind handling.

src/pobsync_backend/tests/test_views.py

@@ -100,6 +100,58 @@ class ViewTests(TestCase):
 
         self.assertEqual(response.status_code, 405)
 
+    def test_retention_plan_renders_keep_and_delete_sets(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,
+        )
+        old_snapshot = self._snapshot(host, "20260518-021500Z__OLDSNAP")
+        new_snapshot = self._snapshot(host, "20260519-021500Z__NEWSNAP")
+
+        response = self.client.get(reverse("host_retention_plan", args=[host.host]))
+
+        self.assertEqual(response.status_code, 200)
+        self.assertContains(response, "Retention Plan: web-01")
+        self.assertContains(response, old_snapshot.dirname)
+        self.assertContains(response, new_snapshot.dirname)
+        self.assertContains(response, "newest")
+        self.assertContains(response, "Would Delete")
+
+    def test_retention_plan_can_enable_base_protection(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,
+        )
+        base = self._snapshot(host, "20260518-021500Z__BASESNAP")
+        child = self._snapshot(host, "20260519-021500Z__CHILDSNP")
+        child.base = base
+        child.save(update_fields=["base"])
+
+        response = self.client.get(reverse("host_retention_plan", args=[host.host]), {"protect_bases": "1"})
+
+        self.assertEqual(response.status_code, 200)
+        self.assertContains(response, "Protect bases:</strong> yes")
+        self.assertContains(response, f"base-of:{child.dirname}")
+
+    def test_retention_plan_rejects_invalid_kind(self) -> None:
+        self.client.force_login(self.staff_user)
+        host = HostConfig.objects.create(host="web-01", address="web-01.example.test")
+
+        response = self.client.get(reverse("host_retention_plan", args=[host.host]), {"kind": "weird"}, follow=True)
+
+        self.assertRedirects(response, reverse("host_detail", args=[host.host]))
+        self.assertContains(response, "Retention kind must be scheduled, manual, or all.")
+
     def _snapshot(self, host: HostConfig, dirname: str) -> SnapshotRecord:
         started_at = datetime.strptime(dirname.split("__", 1)[0], "%Y%m%d-%H%M%SZ").replace(tzinfo=timezone.utc)
         return SnapshotRecord.objects.create(