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)

backend/export.py

@@ -356,7 +356,7 @@ def export_epub(vault_path: Path, file_path: Path) -> bytes:
 
     rendered = _render_body(body, path.parent, vault_path, path)
 
-    uid = f"obsigate-{_slugify(title)}-{int(datetime.datetime.now().timestamp())}"
+    uid = f"obsigate-{_slugify(title)}-{int(datetime.datetime.now(tz=datetime.timezone.utc).timestamp())}"
     xhtml = f"""<?xml version="1.0" encoding="utf-8"?>
 <!DOCTYPE html>
 <html xmlns="http://www.w3.org/1999/xhtml" xml:lang="fr" lang="fr">

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