refactor: extract atomic JSON helpers to io_utils

Centralize read_json/write_json_atomic used by pipeline meta I/O and
worker processed.json bookkeeping. Preserves corrupt-file warning in
load_processed().

Co-authored-by: Cursor <cursoragent@cursor.com>

app/io_utils.py

@@ -0,0 +1,25 @@
+"""Small shared I/O helpers."""
+
+from __future__ import annotations
+
+import json
+from pathlib import Path
+from typing import Any
+
+
+def read_json(path: Path, *, default: Any = None) -> Any:
+    """Read JSON from *path*, returning *default* when missing or corrupt."""
+    if not path.exists():
+        return default
+    try:
+        return json.loads(path.read_text(encoding="utf-8"))
+    except (OSError, ValueError):
+        return default
+
+
+def write_json_atomic(path: Path, data: Any) -> None:
+    """Write JSON via a temp file and atomic replace."""
+    path.parent.mkdir(parents=True, exist_ok=True)
+    tmp = path.with_suffix(path.suffix + ".tmp")
+    tmp.write_text(json.dumps(data, indent=2, ensure_ascii=False), encoding="utf-8")
+    tmp.replace(path)