(feature) Require delete count confirmation for retention apply

Make manual retention application more explicit by requiring operators to
confirm both the host name and the current number of planned deletions.

This reduces the risk of applying a stale or misunderstood retention plan
when the delete set changes between review and confirmation.

src/pobsync_backend/views.py

@@ -542,10 +542,12 @@ def host_retention_plan(request, host: str):
         "scheduled_prune_exceeded": scheduled_prune_limit is not None and delete_count > scheduled_prune_limit,
         "apply_form": RetentionApplyForm(
             host_name=host_config.host,
+            expected_delete_count=delete_count,
             initial={
                 "kind": kind,
                 "protect_bases": protect_bases,
                 "max_delete": delete_count,
+                "confirm_delete_count": delete_count,
             },
         ),
     }
@@ -556,7 +558,26 @@ def host_retention_plan(request, host: str):
 @require_POST
 def apply_host_retention(request, host: str):
     host_config = get_object_or_404(HostConfig, host=host)
-    form = RetentionApplyForm(request.POST, host_name=host_config.host)
+    raw_kind = request.POST.get("kind", "scheduled")
+    raw_protect_bases = request.POST.get("protect_bases") in {"1", "true", "on", "yes"}
+    expected_delete_count = None
+    if raw_kind in {"scheduled", "manual", "all"}:
+        try:
+            plan = run_sql_retention_plan(
+                host=host_config.host,
+                kind=raw_kind,
+                protect_bases=raw_protect_bases,
+            )
+        except PobsyncError as exc:
+            messages.error(request, str(exc))
+            return redirect("host_retention_plan", host=host_config.host)
+        expected_delete_count = len(plan.get("delete") or [])
+
+    form = RetentionApplyForm(
+        request.POST,
+        host_name=host_config.host,
+        expected_delete_count=expected_delete_count,
+    )
     if not form.is_valid():
         messages.error(request, "Retention apply confirmation is invalid.")
         return redirect("host_retention_plan", host=host_config.host)