Add signup form

apps/submissions/signals.py

@@ -0,0 +1,29 @@
+"""Signal handlers for the submissions app.
+
+The cleanup story (plan.md §7.6): when a `Submission` row is deleted --
+manually, via `cleanup_stale`, or for any other reason -- the uploaded
+`.stl` file must also be unlinked from `MEDIA_ROOT`. Django's `FileField`
+does NOT do this automatically (the behaviour was removed in 1.3, ages
+ago), so we wire a `post_delete` signal to take care of it.
+"""
+
+from __future__ import annotations
+
+from django.db.models.signals import post_delete
+from django.dispatch import receiver
+
+from .models import Submission
+
+
+@receiver(post_delete, sender=Submission)
+def unlink_stl_file_on_delete(sender, instance: Submission, **kwargs) -> None:
+    """Remove the on-disk `.stl` after a Submission row is deleted.
+
+    `FileField.delete(save=False)` is idempotent: it just calls
+    `Storage.delete(name)`, and `FileSystemStorage.delete` catches
+    `FileNotFoundError`. Safe to call on rows that never had a file
+    (e.g. URL-source submissions) -- the `if instance.stl_file` truthiness
+    check handles that.
+    """
+    if instance.stl_file:
+        instance.stl_file.delete(save=False)