feat: подписка на уведомление о поступлении товара

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

postgres/init/05_stock_alerts.sql

@@ -0,0 +1,14 @@
+-- Подписка «сообщить о поступлении»
+CREATE TABLE IF NOT EXISTS product_stock_alerts (
+  id SERIAL PRIMARY KEY,
+  product_id INTEGER NOT NULL REFERENCES products(id) ON DELETE CASCADE,
+  email TEXT NOT NULL,
+  user_id INTEGER REFERENCES users(id) ON DELETE SET NULL,
+  created_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+  notified_at TIMESTAMPTZ,
+  UNIQUE (product_id, email)
+);
+
+CREATE INDEX IF NOT EXISTS idx_stock_alerts_product_pending
+  ON product_stock_alerts (product_id)
+  WHERE notified_at IS NULL;