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.

tests/test_modules.py

@@ -277,6 +277,7 @@ class TestWebhooks:
         from backend import webhooks
         self.wh_file = tmp_path / "webhooks.json"
         monkeypatch.setattr(webhooks, "WEBHOOKS_FILE", self.wh_file)
+        monkeypatch.setattr(webhooks, "WEBHOOK_SECRETS_FILE", tmp_path / "webhook_secrets.json")
         yield
 
     def test_get_empty(self):