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/migrations/2026_08_18_000002_add_product_id_to_price_triggers_table.php

@@ -0,0 +1,34 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    public function up(): void
+    {
+        if (! Schema::hasTable('price_triggers')) {
+            return;
+        }
+
+        Schema::table('price_triggers', function (Blueprint $table) {
+            if (! Schema::hasColumn('price_triggers', 'product_id')) {
+                $table->unsignedBigInteger('product_id')->nullable()->after('customer_id');
+                $table->index('product_id');
+            }
+        });
+    }
+
+    public function down(): void
+    {
+        if (! Schema::hasTable('price_triggers') || ! Schema::hasColumn('price_triggers', 'product_id')) {
+            return;
+        }
+
+        Schema::table('price_triggers', function (Blueprint $table) {
+            $table->dropIndex(['product_id']);
+            $table->dropColumn('product_id');
+        });
+    }
+};