feat: persist customer product and shop favorites through the live API

Replace mall fixture favorites with customer-scoped endpoints, and send signed-out shoppers back to the page they left after sign-in.

Co-authored-by: Cursor <cursoragent@cursor.com>

apps/api/migrations/0015_favorites.sql

@@ -0,0 +1,25 @@
+-- Customer favorites: one row per (user, product) or (user, shop).
+-- Two nullable FKs plus a check keep the target shape honest; partial unique
+-- indexes enforce uniqueness without letting NULL shop/product repeat.
+
+CREATE TABLE favorites (
+    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    user_id UUID NOT NULL REFERENCES users (id) ON DELETE CASCADE,
+    product_id UUID REFERENCES products (id) ON DELETE CASCADE,
+    shop_id UUID REFERENCES shops (id) ON DELETE CASCADE,
+    created_at TIMESTAMPTZ NOT NULL DEFAULT now(),
+    CONSTRAINT favorites_exactly_one_target CHECK (
+        (product_id IS NOT NULL AND shop_id IS NULL)
+        OR (product_id IS NULL AND shop_id IS NOT NULL)
+    )
+);
+
+CREATE INDEX favorites_user_created_idx ON favorites (user_id, created_at DESC);
+
+CREATE UNIQUE INDEX favorites_user_product_idx
+    ON favorites (user_id, product_id)
+    WHERE product_id IS NOT NULL;
+
+CREATE UNIQUE INDEX favorites_user_shop_idx
+    ON favorites (user_id, shop_id)
+    WHERE shop_id IS NOT NULL;