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/bookslm.py

@@ -43,9 +43,7 @@ def _should_skip(name: str) -> bool:
     skip_names = {"_attachments", "node_modules", ".git", ".obsidian", "__pycache__"}
     if name in skip_names:
         return True
-    if any(name.startswith(p) for p in skip_prefixes):
-        return True
-    return False
+    return bool(any(name.startswith(p) for p in skip_prefixes))
 
 
 def _file_priority(path: Path) -> tuple[int, float]:
@@ -252,12 +250,7 @@ def invalidate_cache(vault_path: Path | None = None, directory: str | None = Non
         _cache.clear()
         return count
 
-    to_remove = []
-    for key, val in _cache.items():
-        # We can't easily reverse the hash, so clear everything if vault_path is given
-        # For targeted invalidation, callers should use directory
-        to_remove.append(key)
-
+    to_remove = list(_cache)
     for k in to_remove:
         del _cache[k]
     return len(to_remove)