feat: create validated ZUGFeRD invoices in one console command

jr_einvoice/invoice_number.py

@@ -26,3 +26,19 @@ def suggest_next_invoice_number(invoices_root: Path, year: int | None = None, pa
             if yr == y and seq > max_seq:
                 max_seq = seq
     return f"{y}-{max_seq+1:04d}"
+
+
+from contextlib import contextmanager
+
+@contextmanager
+def invoice_root_lock(root: Path):
+    """Serialize local number allocation and folder creation on Linux."""
+    import fcntl
+    root = root.expanduser().resolve()
+    root.mkdir(parents=True, exist_ok=True)
+    with (root / '.jr-einvoice.lock').open('a') as lock:
+        fcntl.flock(lock, fcntl.LOCK_EX)
+        try:
+            yield
+        finally:
+            fcntl.flock(lock, fcntl.LOCK_UN)