fix(planting-service): 使用自定义外观流嵌入签名图片

- setImage 无法正确渲染签名到按钮字段
- 手动创建 XObject Form 外观流
- 计算图片缩放和居中位置
- 设置 widget 的 NormalAppearance

🤖 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

@@ -381,11 +381,55 @@ export class PdfGeneratorService {
         try {
           const form = pdfDoc.getForm();
           const signatureButton = form.getButton(FORM_FIELDS.SIGNATURE);
-          signatureButton.setImage(signatureImage);
-          this.logger.log('Signature embedded using form field');
-        } catch {
+
+          // 获取按钮的 widget 和尺寸
+          const widgets = signatureButton.acroField.getWidgets();
+          if (widgets.length > 0) {
+            const widget = widgets[0];
+            const rect = widget.getRectangle();
+            const { width: fieldWidth, height: fieldHeight } = rect;
+
+            // 计算图片缩放尺寸（保持宽高比，适应字段大小）
+            const imgDims = signatureImage.scale(1);
+            let scale = Math.min(fieldWidth / imgDims.width, fieldHeight / imgDims.height);
+            const scaledWidth = imgDims.width * scale;
+            const scaledHeight = imgDims.height * scale;
+
+            // 计算居中位置
+            const x = (fieldWidth - scaledWidth) / 2;
+            const y = (fieldHeight - scaledHeight) / 2;
+
+            // 创建外观流内容
+            const appearanceStream = `q ${scaledWidth} 0 0 ${scaledHeight} ${x} ${y} cm /Img Do Q`;
+
+            // 创建 XObject Form 作为外观
+            const context = pdfDoc.context;
+            const imageXObjectName = 'Img';
+
+            const Resources = context.obj({
+              XObject: { [imageXObjectName]: signatureImage.ref },
+            });
+
+            const appearanceStreamRef = context.register(
+              context.stream(appearanceStream, {
+                Type: 'XObject',
+                Subtype: 'Form',
+                FormType: 1,
+                BBox: [0, 0, fieldWidth, fieldHeight],
+                Resources,
+              }),
+            );
+
+            // 设置外观到 widget
+            widget.setNormalAppearance(appearanceStreamRef);
+
+            this.logger.log('Signature embedded using custom appearance stream');
+          } else {
+            throw new Error('No widgets found for signature button');
+          }
+        } catch (error) {
           // 如果没有签名按钮字段，回退到坐标方式
-          this.logger.log('Signature button field not found, using coordinates');
+          this.logger.log(`Signature button field error: ${error.message}, using coordinates`);
           this.embedSignatureByCoordinates(pdfDoc, signatureImage);
         }
       }