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

core/print_api.py

@@ -46,6 +46,9 @@ app = FastAPI(
         "when `printer` is omitted.\n"
         "* Paper size and orientation are chosen automatically per report type: "
         "packing_list → A5 landscape, sign_receipt → A4 portrait.\n"
+        "* Packing list supports an A4 carrier mode via `paper=\"a4\"`: the A5 content is "
+        "rendered on an A4 portrait page (top region) and printed as A4, so it comes out "
+        "correctly on A5 paper loaded horizontally in an A4 tray.\n"
         "* A JSON line is appended to `logs/print.log` for every request."
     ),
     version="1.0.0",
@@ -83,6 +86,17 @@ class PrintRequest(BaseModel):
         description="Target printer name. Falls back to `printers.default` in settings.yaml when omitted.",
         examples=["Canon G1030 series (网络 USB1)"],
     )
+    paper: Optional[Literal["a4"]] = Field(
+        None,
+        description=(
+            "Paper carrier mode. Omit (null) for the default A5 landscape packing list. "
+            "Set to \"a4\" to render the packing list on an A4 portrait page with the A5 "
+            "content placed in the top region, so it prints correctly on A5 paper loaded "
+            "horizontally in an A4 tray (saves re-adjusting the paper clamps). Only "
+            "applies to `packing_list`; ignored for other report types."
+        ),
+        examples=["a4"],
+    )
     dry_run: bool = Field(
         False,
         description="When true, generate the PDF and build the print command but do NOT send it to the printer.",
@@ -140,8 +154,9 @@ def _log(result: dict) -> None:
         f.write(json.dumps(entry, ensure_ascii=False, default=str) + "\n")
 
 
-@api_router.post(
-    "/print",
+@app.post(
+    "/api/print",
+    tags=["print"],
     summary="Generate and print a shipping document",
     description=(
         "Generate the requested document as a PDF using the existing report "
@@ -154,7 +169,20 @@ def _log(result: dict) -> None:
         "(skipped when `dry_run=true`).\n"
         "4. Append a JSON line to `logs/print.log`.\n"
         "5. Return the job result.\n\n"
-        "Set `dry_run=true` to generate the PDF without consuming paper."
+        "Set `dry_run=true` to generate the PDF without consuming paper.\n\n"
+        "**A4 Carrier Mode (packing_list only)**\n"
+        "To avoid re-adjusting the printer's paper clamps, A5 paper can be loaded "
+        "**horizontally** in an A4 tray. But horizontal loading would otherwise print "
+        "the A5 content upside-down. This mode fixes it: pass `paper=\"a4\"` and the "
+        "A5 content is rendered into the **top 148mm region of an A4 portrait page**, "
+        "then printed as A4 — the horizontally loaded A5 sheet receives the top region "
+        "and comes out upright and complete. Verified on a real printer.\n\n"
+        "Both conditions are required:\n"
+        "1. Request carries `paper=\"a4\"` (only affects `packing_list`; ignored for others).\n"
+        "2. Physically, the A5 paper is loaded **horizontally** in the A4 tray.\n\n"
+        "For a standard A5 tray (vertical loading), omit `paper` to fall back to normal "
+        "A5 landscape direct print. The A4-carrier PDF is named with an `_a4` suffix; "
+        "always read the returned `pdf` field rather than hard-coding the path."
     ),
     response_description="Print job result, including the generated PDF path.",
     response_model=PrintResponse,
@@ -184,6 +212,7 @@ def print_endpoint(req: PrintRequest):
             params=req.params,
             printer=req.printer,
             dry_run=req.dry_run,
+            paper=req.paper,
         )
     except ValueError as e:
         raise HTTPException(status_code=400, detail=str(e))