fix(security): consolidation & securite phase 1 (#84, BUG-021 a BUG-034)

- sanitizer XSS serveur (markdown + page de partage) [BUG-021/022]
- rate-limit/lockout MFA [BUG-023]
- isolation vaults par segments [BUG-024]
- caps regex ReDoS [BUG-025]
- SSRF webhooks + secrets externalises [BUG-026]
- rotation/revocation des jetons [BUG-027]
- politique de mot de passe + invalidation sessions [BUG-028]
- verrous users.json [BUG-029]
- IP reelle dans les audits [BUG-030]
- rate-limit par compte [BUG-031]
- symlinks hors vault ignores [BUG-032]
- recherche simple via inverted index [BUG-033]
- token en memoire + cookie HttpOnly, CSP durcie [BUG-034]

Tests: pytest 961 passed / 6 skipped, ruff 0, mypy 0, frontend vert.

backend/auth/password.py

@@ -13,6 +13,32 @@ ph = PasswordHasher(
     salt_len=16,
 )
 
+# Password policy (BUG-028). Applied by the API validators at account creation
+# and password change so the rules stay consistent across both paths.
+MIN_PASSWORD_LENGTH = 8
+MAX_PASSWORD_LENGTH = 128
+
+
+def validate_password_strength(password: str) -> str:
+    """Validate a plaintext password against the project policy.
+
+    Args:
+        password: Candidate password.
+
+    Returns:
+        The password unchanged when valid.
+
+    Raises:
+        ValueError: When the password is too short, too long or blank.
+    """
+    if password is None or len(password) < MIN_PASSWORD_LENGTH:
+        raise ValueError(f"Minimum {MIN_PASSWORD_LENGTH} caractères")
+    if len(password) > MAX_PASSWORD_LENGTH:
+        raise ValueError(f"Maximum {MAX_PASSWORD_LENGTH} caractères")
+    if not password.strip():
+        raise ValueError("Le mot de passe ne peut pas être vide")
+    return password
+
 
 def hash_password(password: str) -> str:
     """Hash a password with Argon2id."""