fix(core): stop atomic writes from colliding on a constant PID suffix (#5721)

atomic_write_json/atomic_write_text build their temp filename as
"{path}.tmp.{os.getpid()}". os.getpid() is constant for the life of a
process, so it only ever distinguishes concurrent writers that live in
different OS processes. Odysseus runs as a single long-lived process
per container, so two concurrent writers to the same path (e.g. two
request handlers racing a settings save) always compute the identical
temp path. Whichever finishes os.replace() first removes the shared
tmp file out from under the other, which then raises FileNotFoundError
on its own os.replace() instead of landing its write.

Fix: derive the temp suffix from uuid4() instead of the PID, so every
call gets a distinct temp path regardless of process/thread identity.

routes/prefs_routes.py's _save() had an independent, hand-rolled copy
of the exact same PID-suffix logic (not the shared core.atomic_io
helper other routes already use, e.g. routes/auth_routes.py) with the
same bug. Replaced it with a call to atomic_write_json.

Fixes #5596

Co-authored-by: Alexandre Teixeira <111787685+alteixeira20@users.noreply.github.com>

routes/prefs_routes.py

@@ -1,8 +1,8 @@
 """User preferences API — per-user key/value store backed by a JSON file."""
 import json
-import os
 from typing import Optional
 from fastapi import APIRouter, Request
+from core.atomic_io import atomic_write_json
 from src.auth_helpers import get_current_user
 from src.constants import USER_PREFS_FILE
 
@@ -20,13 +20,7 @@ def _load():
 
 
 def _save(prefs):
-    os.makedirs(os.path.dirname(PREFS_FILE) or ".", exist_ok=True)
-    tmp = f"{PREFS_FILE}.tmp.{os.getpid()}"
-    with open(tmp, "w", encoding="utf-8") as f:
-        json.dump(prefs, f, indent=2)
-        f.flush()
-        os.fsync(f.fileno())
-    os.replace(tmp, PREFS_FILE)
+    atomic_write_json(PREFS_FILE, prefs, indent=2)
 
 
 def _load_for_user(user: Optional[str] = None) -> dict: