(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/forms.py

@@ -249,12 +249,18 @@ class RetentionApplyForm(forms.Form):
     kind = forms.ChoiceField(choices=(("scheduled", "Scheduled"), ("manual", "Manual"), ("all", "All")))
     protect_bases = forms.BooleanField(required=False)
     max_delete = forms.IntegerField(min_value=0, initial=10)
+    confirm_delete_count = forms.IntegerField(min_value=0)
     confirm_host = forms.CharField()
 
-    def __init__(self, *args, host_name: str, **kwargs) -> None:
+    def __init__(self, *args, host_name: str, expected_delete_count: int | None = None, **kwargs) -> None:
         self.host_name = host_name
+        self.expected_delete_count = expected_delete_count
         super().__init__(*args, **kwargs)
         self.fields["confirm_host"].help_text = f"Type {host_name} to confirm deletion."
+        if expected_delete_count is not None:
+            self.fields["confirm_delete_count"].help_text = (
+                f"Type {expected_delete_count} to confirm the current number of planned deletions."
+            )
 
     def clean_confirm_host(self) -> str:
         value = self.cleaned_data["confirm_host"].strip()
@@ -262,6 +268,12 @@ class RetentionApplyForm(forms.Form):
             raise forms.ValidationError(f"Type {self.host_name} to confirm.")
         return value
 
+    def clean_confirm_delete_count(self) -> int:
+        value = self.cleaned_data["confirm_delete_count"]
+        if self.expected_delete_count is not None and value != self.expected_delete_count:
+            raise forms.ValidationError(f"Type {self.expected_delete_count} to confirm the delete count.")
+        return value
+
 
 class ScheduleConfigForm(forms.ModelForm):
     cron_expr = forms.CharField(