feat(schema): ingredient.section_heading (Migration 012 + Type)

Fuegt das nullable Feld section_heading zur ingredient-Tabelle hinzu
(Migration 012), erweitert den Ingredient-Typ und aktualisiert alle drei
Return-Stellen in parseIngredient. Downstream-Sites (repository, Editor,
Tests) bleiben rot – werden in Task 2+ behoben.

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

src/lib/server/db/migrations/012_ingredient_section.sql

@@ -0,0 +1,4 @@
+-- Nullable -- old rows keep NULL, new rows may have a section heading.
+-- Rendering rule: if section_heading is set (not NULL and not empty),
+-- a new section with this title starts at this ingredient row.
+ALTER TABLE ingredient ADD COLUMN section_heading TEXT;