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>

supabase/migrations/006_item_frequency.sql

@@ -0,0 +1,70 @@
+-- Migration 006: item_frequency — per-collective item usage tracking.
+--
+-- Write path: SECURITY DEFINER trigger only. The app role has SELECT access only.
+-- Names are normalized (lower + trim) before upsert so "Milk" and "milk" are the same entry.
+
+-- ── item_frequency ────────────────────────────────────────────────────────────
+
+CREATE TABLE public.item_frequency (
+    collective_id uuid        NOT NULL REFERENCES public.collectives(id) ON DELETE CASCADE,
+    name          text        NOT NULL,   -- normalized: lower(trim(original name))
+    use_count     integer     NOT NULL DEFAULT 1,
+    last_used_at  timestamptz NOT NULL DEFAULT now(),
+    PRIMARY KEY (collective_id, name)
+);
+
+ALTER TABLE public.item_frequency ENABLE ROW LEVEL SECURITY;
+
+-- Index for the suggestion query: ordered by use_count descending per collective
+CREATE INDEX item_frequency_collective_count_idx
+    ON public.item_frequency (collective_id, use_count DESC);
+
+-- ── RLS ───────────────────────────────────────────────────────────────────────
+
+-- Members (any role) can read frequency data for collectives they belong to.
+CREATE POLICY item_frequency_select
+    ON public.item_frequency FOR SELECT
+    USING (public.is_member(collective_id));
+
+-- All direct writes are blocked; the SECURITY DEFINER trigger below is the only
+-- write path. This prevents clients from inflating suggestion scores artificially.
+CREATE POLICY item_frequency_insert_deny
+    ON public.item_frequency FOR INSERT
+    WITH CHECK (false);
+
+CREATE POLICY item_frequency_update_deny
+    ON public.item_frequency FOR UPDATE
+    USING (false);
+
+CREATE POLICY item_frequency_delete_deny
+    ON public.item_frequency FOR DELETE
+    USING (false);
+
+-- ── Trigger: upsert frequency on shopping_items insert ───────────────────────
+
+CREATE OR REPLACE FUNCTION public.fn_record_item_frequency()
+RETURNS TRIGGER
+LANGUAGE plpgsql
+SECURITY DEFINER
+SET search_path = public
+AS $$
+DECLARE
+    v_collective_id uuid;
+BEGIN
+    SELECT collective_id INTO v_collective_id
+    FROM   public.shopping_lists
+    WHERE  id = NEW.list_id;
+
+    INSERT INTO public.item_frequency (collective_id, name, use_count, last_used_at)
+    VALUES (v_collective_id, lower(trim(NEW.name)), 1, now())
+    ON CONFLICT (collective_id, name) DO UPDATE SET
+        use_count    = item_frequency.use_count + 1,
+        last_used_at = now();
+
+    RETURN NEW;
+END;
+$$;
+
+CREATE TRIGGER trg_item_frequency
+    AFTER INSERT ON public.shopping_items
+    FOR EACH ROW EXECUTE FUNCTION public.fn_record_item_frequency();