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

reports/packing_list/_build_template.py

@@ -124,10 +124,18 @@ def styles() -> list[dict]:
     ]
 
 
-def document_properties() -> dict:
+def document_properties(paper: str = "a5") -> dict:
     # 边距按 pt 给出（ReportBro 不换算 margin；pageFormat=A5 自动换算页面尺寸为 pt）。
+    # A4 承载模式：物理页面仍是 A4 纵向(210x297mm)，但内容按 A5 横向(210x148mm)设计、
+    # 自然落在页面顶部 148mm 区域。配合打印机把 A5 纸横向装载进 A4 纸盒，发送 A4 指令即可在
+    # 横向 A5 纸上正常出纸，省去反复调整不同幅面纸盒卡扣的过程。
+    # 因 A5 横向与 A4 纵向宽度同为 210mm，元素 x/宽度完全不变，仅画面高度由 148→297mm。
+    if paper == "a4":
+        fmt, orient = "A4", "portrait"
+    else:
+        fmt, orient = "A5", "landscape"
     return {
-        "pageFormat": "A5", "orientation": "landscape",
+        "pageFormat": fmt, "orientation": orient,
         "marginLeft": mm(MARGIN_L), "marginRight": mm(MARGIN_R),
         "marginTop": mm(10), "marginBottom": mm(10),
         "headerDisplay": "never", "headerSize": 0,
@@ -324,11 +332,14 @@ def build_doc_elements(context: dict[str, Any]) -> list[dict]:
     return els
 
 
-def build_report_definition(context: dict[str, Any]) -> dict:
-    """组装完整 report_definition（运行时调用，按数据动态布局）。"""
+def build_report_definition(context: dict[str, Any], paper: str = "a5") -> dict:
+    """组装完整 report_definition（运行时调用，按数据动态布局）。
+
+    :param paper: "a5"（默认，A5 横向直打）或 "a4"（A4 纵向承载模式）。
+    """
     return {
         "version": 6,
-        "documentProperties": document_properties(),
+        "documentProperties": document_properties(paper),
         "parameters": parameters(),
         "styles": styles(),
         "docElements": build_doc_elements(context),