draft mode + homework form consolidation

[PavanVemparala]

Draft mode for homework assignments — teachers can now save incomplete homeworks as drafts and publish when
ready, with full scheduling and expiry support.

Core changes

Model (homeworks/models.py)

• due_date made nullable — drafts no longer require a due date
• Added homework_type (draft/published/hidden) as a display label
• Added publish_at — datetime at which a draft auto-publishes
• is_hidden remains the single source of truth for student access (never homework_type)
• New properties: is_draft, should_auto_publish; is_overdue guarded against None due_date

Publishing flow (homeworks/forms.py, views)

• Removed is_hidden from both forms — it is now managed automatically by the server
• Added "Publish now" toggle (checked by default): when on, publishes immediately; when off, publish_at is
  required and must be future
• Save as Draft bypasses all validation — only title/description written directly to ORM
• Scheduled publish: saving with publish_now unchecked keeps homework as DRAFT with publish_at set;
  auto_publish_due_drafts() runs lazily on each page load to promote these
• Re-publishing an expired homework automatically clears a stale expires_at

Section UX (homeworks/templates/homeworks/form.html)

• order field replaced with up/down arrow buttons — order managed automatically by JS
• "Add Another Section" now clones from a permanent hidden template, fixing a crash when the section container
  is empty (e.g. editing a draft with no sections yet)

Course detail (courses/views.py, courses/templates/courses/detail.html)

• Homework list shows draft/hidden/expired/scheduled badges matching the homeworks list view
• Duplicate HomeworkFormData dataclass removed — CourseHomeworkCreateView now shares the same form template and dataclass as HomeworkEditView