fix(planting-service): 签名图片按比例缩放到合适大小

- 目标宽度 150pt（约 5cm）
- 保持宽高比不变
- 避免签名图片过大

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/services/planting-service/src/infrastructure/pdf/pdf-generator.service.ts

@@ -388,25 +388,28 @@ export class PdfGeneratorService {
             const widget = widgets[0];
             const rect = widget.getRectangle();
 
-            // 获取签名图片原始尺寸
+            // 获取签名图片原始尺寸并按比例缩放
             const imgDims = signatureImage.scale(1);
-            const imgWidth = imgDims.width;
-            const imgHeight = imgDims.height;
 
-            // 直接在按钮位置绘制原始尺寸的签名图片
+            // 目标宽度 150pt（约 5cm），保持宽高比
+            const targetWidth = 150;
+            const scale = targetWidth / imgDims.width;
+            const scaledWidth = targetWidth;
+            const scaledHeight = imgDims.height * scale;
+
             // 获取按钮所在的页面
             const pages = pdfDoc.getPages();
             const page = pages[5]; // 第6页（签名区域）
 
-            // 在按钮的左下角位置绘制签名图片（保持原始尺寸）
+            // 在按钮位置绘制缩放后的签名图片
             page.drawImage(signatureImage, {
               x: rect.x,
               y: rect.y,
-              width: imgWidth,
-              height: imgHeight,
+              width: scaledWidth,
+              height: scaledHeight,
             });
 
-            this.logger.log(`Signature embedded at (${rect.x}, ${rect.y}) with original size ${imgWidth}x${imgHeight}`);
+            this.logger.log(`Signature embedded at (${rect.x}, ${rect.y}) with size ${scaledWidth}x${scaledHeight}`);
           } else {
             throw new Error('No widgets found for signature button');
           }