fix(purchase): year_month 예약어 백틱 처리

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

system1-factory/api/db/migrations/20260313100000_create_purchase_system.sql

@@ -86,7 +86,7 @@ CREATE TABLE IF NOT EXISTS consumable_price_history (
 -- 월간 정산
 CREATE TABLE IF NOT EXISTS monthly_settlements (
     settlement_id INT AUTO_INCREMENT PRIMARY KEY,
-    year_month VARCHAR(7) NOT NULL COMMENT 'YYYY-MM',
+    `year_month` VARCHAR(7) NOT NULL COMMENT 'YYYY-MM',
     vendor_id INT NOT NULL,
     total_amount DECIMAL(12,0) DEFAULT 0,
     status ENUM('pending','completed') DEFAULT 'pending',
@@ -96,7 +96,7 @@ CREATE TABLE IF NOT EXISTS monthly_settlements (
     created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
     updated_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
     FOREIGN KEY (vendor_id) REFERENCES vendors(vendor_id),
-    UNIQUE KEY uq_ym_vendor (year_month, vendor_id)
+    UNIQUE KEY uq_ym_vendor (`year_month`, vendor_id)
 );
 
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