Handle corrupt/empty notebook files, atomic writes

_read_notebook_file returns None on empty or unparseable JSON instead
of crashing with JSONDecodeError (surfaced as 500). save_notebook now
writes to a temp file and atomically renames, preventing empty files
from container restarts mid-write.

backend/src/spicebook/storage/filesystem.py

@@ -2,7 +2,9 @@
 
 import json
 import logging
+import os
 import re
+import tempfile
 import uuid
 from datetime import datetime, timezone
 from pathlib import Path
@@ -116,6 +118,9 @@ def list_notebooks(directory: Path) -> list[NotebookSummary]:
 
             try:
                 nb = _read_notebook_file(path)
+                if nb is None:
+                    logger.warning("Skipping empty/corrupt notebook: %s", path)
+                    continue
                 description, schematic_svg = _extract_card_data(nb)
                 summaries.append(NotebookSummary(
                     id=nb_id,
@@ -152,10 +157,18 @@ def save_notebook(directory: Path, notebook_id: str, notebook: Notebook) -> None
     notebook.metadata.modified = _now_iso()
 
     path = user_dir / f"{notebook_id}{SPICEBOOK_EXT}"
-    path.write_text(
-        notebook.model_dump_json(indent=2),
-        encoding="utf-8",
-    )
+    content = notebook.model_dump_json(indent=2)
+    # Atomic write: write to temp file then rename, so a crash can't leave an empty file
+    fd, tmp = tempfile.mkstemp(dir=user_dir, suffix=".tmp")
+    try:
+        os.write(fd, content.encode("utf-8"))
+        os.fsync(fd)
+        os.close(fd)
+        os.replace(tmp, path)
+    except BaseException:
+        os.close(fd)
+        os.unlink(tmp)
+        raise
 
 
 def create_notebook(directory: Path, title: str, engine: str = "ngspice") -> tuple[str, Notebook]:
@@ -196,8 +209,13 @@ def delete_notebook(directory: Path, notebook_id: str) -> bool:
     return False
 
 
-def _read_notebook_file(path: Path) -> Notebook:
-    """Read and validate a .spicebook JSON file."""
+def _read_notebook_file(path: Path) -> Notebook | None:
+    """Read and validate a .spicebook JSON file. Returns None on corrupt/empty files."""
     raw = path.read_text(encoding="utf-8")
-    data = json.loads(raw)
+    if not raw.strip():
+        return None
+    try:
+        data = json.loads(raw)
+    except json.JSONDecodeError:
+        return None
     return Notebook.model_validate(data)