fix(planting): remove signature button field before flatten to avoid gray background

The signature button field has a gray background that covers the drawn
signature image when the form is flattened. Now we remove the signature
field after drawing the signature image to prevent this.

🤖 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

@@ -442,6 +442,16 @@ export class PdfGeneratorService {
         });
 
         this.logger.log(`Signature drawn at (${signatureX}, ${signatureY}), size: ${scaledWidth}x${scaledHeight}`);
+
+        // 移除签名按钮字段，避免扁平化时产生灰色背景
+        try {
+          const form = pdfDoc.getForm();
+          const signatureButton = form.getButton(FORM_FIELDS.SIGNATURE);
+          form.removeField(signatureButton);
+          this.logger.log('Signature button field removed');
+        } catch {
+          // 忽略移除失败
+        }
       }
 
       // 6. 最后统一扁平化所有表单字段