feat: Fase 2a — shopping lists CRUD
DB: shopping_lists + shopping_items tables with full RLS, pg_cron jobs
(archive completed lists after 7 days, purge trash after 7 days).
item_frequency table with SECURITY DEFINER trigger that normalises and
upserts on every shopping_items INSERT.
Frontend: /lists overview (featured card + 2-col grid + trash toggle),
/lists/[id] detail (qty stepper, inline edit, swipe-to-reveal-delete,
drag & drop reorder via svelte-dnd-action, ItemSuggestions chips).
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>