Add HTTP print API for packing list and sign-off receipt

- core/printing.py: SumatraPDF silent print backend (dry_run, timeout, error capture)
- core/print_document.py: orchestrate generate() + print, per-report paper size, printer fallback
- core/print_api.py: FastAPI POST /api/print + GET /api/health, HTTPBearer auth, English Swagger at /api/docs
- serve.py: uvicorn entrypoint
- config: printers/print/api sections in settings.yaml + core/settings.py
- requirements.txt: add fastapi, uvicorn
- .gitignore: ignore logs/

core/printing.py

@@ -0,0 +1,64 @@
+"""打印后端：把已生成的 PDF 静默送到物理/网络打印机。
+
+MVP 使用 SumatraPDF 命令行打印（轻量、支持 -print-to 指定打印机与纸张，
+配合 -silent 适合服务端无人值守打印）。
+"""
+from __future__ import annotations
+
+import subprocess
+from pathlib import Path
+
+
+def build_command(
+    sumatrapdf_path: str, printer: str, settings: str, pdf_path: str
+) -> list[str]:
+    """构造 SumatraPDF 打印命令。"""
+    return [
+        sumatrapdf_path,
+        "-print-to",
+        printer,
+        "-print-settings",
+        settings,
+        "-silent",
+        pdf_path,
+    ]
+
+
+def print_pdf(
+    pdf_path: str | Path,
+    printer: str,
+    settings: str = "paper=A4,orientation=portrait,color=bw",
+    sumatrapdf_path: str = "C:/Program Files/SumatraPDF/SumatraPDF.exe",
+    timeout: int = 60,
+    dry_run: bool = False,
+) -> dict:
+    """调用 SumatraPDF 把 PDF 静默打印到指定打印机。
+
+    返回结构化结果；dry_run=True 时只构造命令并上报，不真正执行（用于验证链路）。
+    """
+    pdf_path = str(pdf_path)
+    cmd = build_command(sumatrapdf_path, printer, settings, pdf_path)
+    result: dict = {
+        "printer": printer,
+        "pdf": pdf_path,
+        "command": cmd,
+        "dry_run": dry_run,
+    }
+    if dry_run:
+        result["ok"] = True
+        result["note"] = "dry_run: command constructed, not executed"
+        return result
+    try:
+        proc = subprocess.run(
+            cmd, capture_output=True, text=True, timeout=timeout, check=False
+        )
+        result["returncode"] = proc.returncode
+        result["stdout"] = proc.stdout
+        result["stderr"] = proc.stderr
+        result["ok"] = proc.returncode == 0
+        if proc.returncode != 0:
+            result["error"] = (proc.stderr or proc.stdout or "unknown error").strip()
+    except subprocess.TimeoutExpired:
+        result["ok"] = False
+        result["error"] = f"print timeout after {timeout}s"
+    return result