chore(lint): ruff --fix sur le backend (cleanup pré-existant)

Réduit les erreurs ruff de 11 à 5 (toutes pré-existantes non auto-fixables) :
- F401 imports inutilisés dans auth/mfa.py
- I001 blocs d'imports non triés dans auth/router.py + main.py + pdf_reader.py

Les 5 restantes sont dans bookslm/export/watcher (code pré-existant, hors scope).

Vérifié : pytest full suite reste 492 passed, 5 skipped, 0 failed.

backend/auth/mfa.py

@@ -8,8 +8,6 @@ import secrets
 
 import pyotp
 
-from .password import hash_password, verify_password
-
 logger = logging.getLogger("obsigate.auth.mfa")
 
 TOTP_ISSUER = "ObsiGate"

backend/auth/router.py

@@ -20,7 +20,6 @@ from .jwt_handler import (
     is_token_revoked,
     revoke_token,
 )
-from .middleware import is_auth_enabled, require_admin, require_auth
 from .mfa import (
     generate_qr_uri,
     generate_recovery_codes,
@@ -29,6 +28,7 @@ from .mfa import (
     verify_recovery_code,
     verify_totp,
 )
+from .middleware import is_auth_enabled, require_admin, require_auth
 from .password import hash_password, verify_password
 from .user_store import (
     create_user,

backend/main.py

@@ -693,9 +693,9 @@ except Exception:  # pragma: no cover - WeasyPrint/GTK missing
     logging.getLogger("obsigate").warning("PDF export unavailable (WeasyPrint/GTK not found)")
 
 # Multi-format export (HTML / MD bundle / ePub) — pure Python, no heavy deps.
-from backend.export import export_epub, export_html, export_md_bundle, ExportError  # noqa: E402
 from backend.ai_routes import router as ai_router
 from backend.bookslm_routes import router as bookslm_router
+from backend.export import ExportError, export_epub, export_html, export_md_bundle
 from backend.saved_searches import delete_saved, get_saved, save_search
 from backend.share import (
     create_share,

backend/pdf_reader.py

@@ -14,7 +14,7 @@ try:
     logger.info("PDF reader: pymupdf (high performance)")
 except ImportError:
     try:
-        from pypdf import PdfReader  # type: ignore  # noqa: F811
+        from pypdf import PdfReader  # type: ignore
         logger.info("PDF reader: pypdf (pure Python)")
     except ImportError:
         logger.warning("No PDF reader available — install pypdf or pymupdf")