(release) Add explicit incomplete snapshot cleanup

Add a dedicated cleanup path for incomplete snapshots instead of letting
retention prune them implicitly. The retention plan now exposes a guarded
form that requires host and delete-count confirmation before removing
.incomplete snapshot directories and their SQL records.

Keep scheduled/manual retention behavior unchanged, add path safety checks,
and cover cleanup success, confirmation failures, max-delete limits, and
unexpected paths in tests.

Refs #10

src/pobsync_backend/forms.py

@@ -274,6 +274,36 @@ class RetentionApplyForm(forms.Form):
         return value
 
 
+class IncompleteCleanupForm(forms.Form):
+    max_delete = forms.IntegerField(min_value=0, initial=0)
+    confirm_delete_count = forms.IntegerField(min_value=0)
+    confirm_host = forms.CharField()
+
+    def __init__(self, *args, host_name: str, expected_delete_count: int, **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 incomplete snapshot cleanup."
+        self.fields["confirm_delete_count"].help_text = (
+            f"Type {expected_delete_count} to confirm the current number of incomplete snapshots."
+        )
+        self.fields["max_delete"].help_text = (
+            f"Must be at least {expected_delete_count} for the incomplete snapshots shown here."
+        )
+
+    def clean_confirm_host(self) -> str:
+        value = self.cleaned_data["confirm_host"].strip()
+        if value != self.host_name:
+            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 value != self.expected_delete_count:
+            raise forms.ValidationError(f"Type {self.expected_delete_count} to confirm the incomplete count.")
+        return value
+
+
 class ScheduleConfigForm(forms.ModelForm):
     cron_expr = forms.CharField(
         label="Schedule expression",