Add A4 carrier print mode (paper=a4) and document it in README + Swagger

- packing_list renders A5 content into top region of an A4 portrait page
  when paper=a4, so A5 paper loaded horizontally in an A4 tray prints upright
- run.generate/print_document/print_api thread the optional paper param through
- README: new Print Service section with A4 carrier usage + curl example
- Swagger: POST /api/print description documents the A4 carrier mode

run.py

@@ -15,6 +15,7 @@ from __future__ import annotations
 
 import argparse
 import importlib
+import inspect
 import json
 import sys
 from datetime import datetime
@@ -80,8 +81,12 @@ def _default_output(report_name: str, params: dict) -> str:
     return f"out/{report_name}.pdf"
 
 
-def generate(report_name: str, params: dict, output: str) -> Path:
-    """生成指定报表的 PDF。"""
+def generate(report_name: str, params: dict, output: str, paper: str | None = None) -> Path:
+    """生成指定报表的 PDF。
+
+    :param paper: 可选纸张承载模式（如 "a4"）。透传给模板的 build_report_definition；
+                  模板不接收该参数时（如签收单）自动忽略，不影响原有渲染。
+    """
     from reportbro import Report  # 延迟导入，CLI 报错时信息更清晰
 
     report_dir = PROJECT_ROOT / "reports" / report_name
@@ -104,7 +109,10 @@ def generate(report_name: str, params: dict, output: str) -> Path:
 
     # 3. 渲染：模板元素按数据动态布局（明细行高度自适应）
     layout = importlib.import_module(f"reports.{report_name}._build_template")
-    report_def = layout.build_report_definition(context)
+    if "paper" in inspect.signature(layout.build_report_definition).parameters:
+        report_def = layout.build_report_definition(context, paper=paper)
+    else:
+        report_def = layout.build_report_definition(context)
     report = Report(
         report_definition=report_def,
         data=context,