feat(server): evaluate price alerts per product and fix gold discharge rounding

Alerts now bind to a product_id and share one evaluator; displayed gold within a rounding step discharges the full wallet.

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

database/schema/mysql-schema.sql.reference

@@ -593,6 +593,7 @@ DROP TABLE IF EXISTS `price_triggers`;
 CREATE TABLE `price_triggers` (
   `id` bigint unsigned NOT NULL AUTO_INCREMENT,
   `customer_id` bigint unsigned NOT NULL,
+  `product_id` bigint unsigned DEFAULT NULL,
   `target_price` bigint NOT NULL,
   `price_condition` varchar(50) CHARACTER SET latin1 COLLATE latin1_swedish_ci NOT NULL,
   `action` varchar(50) CHARACTER SET latin1 COLLATE latin1_swedish_ci NOT NULL,