Backend: product sub-choices, sort_order, preference shared_subset, hard-delete

- ProductOption and ProductPreferenceChoice gain sub_choices (JSON Text column)
  for nested inline choices shown when the parent is selected
- ProductPreferenceSet gains default_choice_id and shared_subset (set-level
  sub-choice group shown for all choices that don't disable it)
- Product gains sort_order column; list endpoint orders by sort_order
- New PUT /products/reorder endpoint for drag-and-drop ordering
- DELETE /products/{id} now accepts ?hard=true for permanent deletion (blocked
  if product appears in any past order)
- Schemas updated with model_validators to parse stored JSON back to typed objects
- Add python-multipart to requirements (needed for file upload form parsing)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

local_backend/requirements.txt

@@ -7,3 +7,4 @@ Pillow==10.4.0
 bcrypt==4.2.0
 pyjwt==2.9.0
 httpx==0.27.2
+python-multipart==0.0.9