feat: 백제약품 잔고 API 엔드포인트 추가

GET /api/baekje/balance - 잔고액 조회

backend/baekje_api.py

@@ -233,3 +233,30 @@ def api_search_for_order():
         result['count'] = len(filtered)
     
     return jsonify(result)
+
+
+# ========== 잔고 조회 ==========
+
+@baekje_bp.route('/balance', methods=['GET'])
+def api_get_balance():
+    """
+    잔고액 조회
+    
+    GET /api/baekje/balance
+    GET /api/baekje/balance?year=2026
+    
+    Returns:
+        {
+            "success": true,
+            "balance": 14193234,
+            "monthly": [
+                {"month": "2026-03", "sales": 6935133, "balance": 14193234, ...},
+                {"month": "2026-02", "sales": 18600692, "balance": 7258101, ...}
+            ]
+        }
+    """
+    year = flask_request.args.get('year', '').strip()
+    
+    session = get_baekje_session()
+    result = session.get_balance(year if year else None)
+    return jsonify(result)