v0.3.1

Fixed

• Deleting a food or unit that is still on a recipe or shopping list now
  answers with an error naming the way out. Mealie deletes the row directly,
  so a reference from anywhere still pointing at it comes back as a foreign
  key violation, and nothing in this server can repoint those rows. The raw
  "Mealie returned 409: ForeignKeyViolation" said nothing about that; the
  error now points at merge for the resources that support it. docs/HOWTO.md
  also now notes that library_stats counts recipe usage only, so a food it
  reports as unused may still be sitting on somebody's shopping list.
• The update_recipe docstring named the wrong field for the slug to use
  after a rename. "The result carries the new slug plus renamed_from — use
  that slug" pointed at renamed_from, its nearest antecedent, which is the
  old slug and a 404 from then on. It now says to use slug, the live one.