chore(lint): fixe les 3 dernières erreurs ruff pré-existantes

- bookslm.py: PERF402 — list(_cache) au lieu de la boucle d'append
- bookslm_routes.py: SIM102 — combine les conditions imbriquées (via --unsafe-fixes)
- export.py: DTZ005 — datetime.now(tz=timezone.utc) au lieu de naive now()
- watcher.py: SIM102 — combine les conditions imbriquées

Résultat : ruff check backend/ → All checks passed!
Vérifié : pytest full suite reste 492 passed, 5 skipped, 0 failed.

backend/watcher.py

@@ -39,10 +39,9 @@ def find_mount_fstype(resolved_path: str, mount_lines: Iterable[str]) -> str:
         # /proc/mounts escapes special chars in octal
         mount_point = mount_point.replace("\\040", " ").replace("\\011", "\t")
         root = mount_point.rstrip("/") or "/"
-        if resolved_path == root or resolved_path.startswith(root + "/"):
-            if len(mount_point) > len(best_point):
-                best_point = mount_point
-                best_fstype = fstype
+        if (resolved_path == root or resolved_path.startswith(root + "/")) and len(mount_point) > len(best_point):
+            best_point = mount_point
+            best_fstype = fstype
     return best_fstype