fix(pdf-generator): 签名按字段尺寸等比例缩放并居中

- 计算宽度和高度的缩放比例，取较小值确保签名完全在字段内
- 在字段内居中放置签名图片
- 符合行业标准：签名根据字段尺寸自适应缩放

🤖 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

@@ -388,28 +388,35 @@ export class PdfGeneratorService {
             const widget = widgets[0];
             const rect = widget.getRectangle();
 
-            // 获取签名图片原始尺寸并按比例缩放
+            // 获取签名图片原始尺寸
             const imgDims = signatureImage.scale(1);
 
-            // 目标宽度 150pt（约 5cm），保持宽高比
-            const targetWidth = 150;
-            const scale = targetWidth / imgDims.width;
-            const scaledWidth = targetWidth;
+            // 按字段尺寸等比例缩放签名图片
+            // 计算宽度和高度的缩放比例，取较小值以确保签名完全在字段内
+            const scaleX = rect.width / imgDims.width;
+            const scaleY = rect.height / imgDims.height;
+            const scale = Math.min(scaleX, scaleY);
+
+            const scaledWidth = imgDims.width * scale;
             const scaledHeight = imgDims.height * scale;
 
+            // 在字段内居中放置签名
+            const offsetX = (rect.width - scaledWidth) / 2;
+            const offsetY = (rect.height - scaledHeight) / 2;
+
             // 获取按钮所在的页面
             const pages = pdfDoc.getPages();
             const page = pages[5]; // 第6页（签名区域）
 
-            // 在按钮位置绘制缩放后的签名图片
+            // 在按钮位置绘制缩放后的签名图片（居中）
             page.drawImage(signatureImage, {
-              x: rect.x,
-              y: rect.y,
+              x: rect.x + offsetX,
+              y: rect.y + offsetY,
               width: scaledWidth,
               height: scaledHeight,
             });
 
-            this.logger.log(`Signature embedded at (${rect.x}, ${rect.y}) with size ${scaledWidth}x${scaledHeight}`);
+            this.logger.log(`Signature embedded at (${rect.x + offsetX}, ${rect.y + offsetY}) with size ${scaledWidth}x${scaledHeight}, field size: ${rect.width}x${rect.height}`);
           } else {
             throw new Error('No widgets found for signature button');
           }