feat(auth): B-2.6 WebAuthn / Passkey support (FIDO2 + biometric 2FA)

Adds phishing-resistant 2nd factor via FIDO2 hardware keys (YubiKey etc.)
and device biometrics (Touch ID, Windows Hello, etc.). Reuses the existing
B-2.5 TOTP gate so a passkey is a 3rd valid option on /2fa/verify, alongside
TOTP code and recovery code. Post-login enrolment lives at /2fa/passkey/setup.

Wraps python-webauthn==2.5.2 in a thin service layer (src/auth/webauthn.py)
that persists credentials in the existing User.webauthn_credentials JSON
column (added in B-2.1 — no schema change). Each credential dict carries
id, public_key, sign_count, transports, name, and created_at. sign_count is
updated after every successful authentication for WebAuthn anti-cloning
(§6.1.1).

Backend: 6 new auth routes (passkey_setup, register/begin, register/finish,
delete, auth/begin, auth/finish). The 4 JSON endpoints are CSRF-exempt at
Flask-WTF level because CSRFProtect cannot read tokens from a JSON body
without app-wide config; the X-CSRFToken header is still sent as
defence-in-depth. The form-POST delete route DOES enforce CSRF. The
@csrf_exempt decorator was previously a no-op label; init_auth_extensions
now walks module-level functions and applies real csrf.exempt() to any
flagged with _csrf_exempt=True.

Login gate now fires when the user has TOTP enabled OR at least one
passkey, and totp_verify_login passes has_passkeys + has_totp flags so the
template can show only the relevant sections.

Frontend: templates/auth/totp_verify.html updated IN PLACE with a passkey
button section (above TOTP) and an "ou" divider. New
templates/auth/passkey_setup.html for managing/enrolling passkeys. New
static/js/webauthn-client.js (no external deps, ES2020) wraps
navigator.credentials and exchanges base64url payloads with the backend.
Tailwind CSS rebuilt.

Tests: 22 new tests in tests/test_webauthn_passkey.py covering the service
layer (b64url helpers, RP config, list/has, begin/finish for both
registration and authentication, delete) and the route flow (CSRF-exempt
JSON endpoints, login gate redirection, sign_count anti-cloning
persistence). Mocks python-webauthn's verify_* functions so tests run
without a real authenticator. Windows manual driver follows the existing
no-conftest pattern.

Self-review: 22/22 new tests pass; 21/21 prior TOTP, 16/16 email,
21/21 OAuth tests still pass (no regression).

Env: config/env.oauth.example documents WEBAUTHN_RP_ID, WEBAUTHN_RP_NAME,
WEBAUTHN_ORIGIN with full deployment notes.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

templates/auth/totp_verify.html

@@ -27,6 +27,24 @@
       <div role="alert" class="mb-4 p-3 rounded-lg text-sm bg-red-50 text-red-900 border border-red-200">{{ error }}</div>
     {% endif %}
 
+    {# B-2.6: Passkey path (only if user has at least one registered passkey) #}
+    {% if has_passkeys %}
+    <section class="mb-6" aria-labelledby="passkey-section-title">
+      <h2 id="passkey-section-title" class="text-base font-semibold text-brand-navy mb-3">Connexion par Passkey</h2>
+      <button id="passkey-auth-btn" type="button" class="w-full grad-bg text-white font-semibold py-3 rounded-[0.75rem] shadow-cta hover:shadow-cta-hover transition focus-visible:outline-2 focus-visible:outline-brand-b1 focus-visible:outline-offset-2">
+        Utiliser ma Passkey
+      </button>
+      <p id="passkey-status" class="text-xs text-brand-navy/70 mt-2" role="status" aria-live="polite"></p>
+    </section>
+
+    {% if has_totp %}
+    <div class="my-4 flex items-center gap-3 text-xs uppercase tracking-wider text-brand-navy/50" aria-hidden="true">
+      <span class="flex-1 h-px bg-brand-border"></span><span>ou</span><span class="flex-1 h-px bg-brand-border"></span>
+    </div>
+    {% endif %}
+    {% endif %}
+
+    {% if has_totp %}
     {# Primary path: 6-digit TOTP code #}
     <form method="POST" action="{{ url_for('auth.totp_verify_login') }}" class="space-y-4" novalidate>
       <input type="hidden" name="csrf_token" value="{{ csrf_token() }}">
@@ -66,6 +84,7 @@
         <p class="text-xs text-brand-navy/60 text-center" aria-live="polite">{{ recovery_codes_remaining }} code{{ 's' if recovery_codes_remaining != 1 else '' }} de récupération restant{{ 's' if recovery_codes_remaining != 1 else '' }}.</p>
       </form>
     </details>
+    {% endif %}
 
     <p class="text-center text-sm text-brand-navy/70 mt-6 pt-4 border-t border-brand-border">
       <a href="{{ url_for('auth.logout') }}" class="grad-text font-semibold">Annuler la connexion</a>
@@ -73,3 +92,20 @@
   </div>
 </section>
 {% endblock %}
+
+{% block scripts %}
+{% if has_passkeys %}
+<script src="{{ url_for('static', filename='js/webauthn-client.js') }}"></script>
+<script>
+  if (window.DictIAWebAuthn) {
+    window.DictIAWebAuthn.wireAuthButton({
+      buttonId: 'passkey-auth-btn',
+      statusElementId: 'passkey-status',
+      beginUrl: '{{ url_for("auth.passkey_auth_begin") }}',
+      finishUrl: '{{ url_for("auth.passkey_auth_finish") }}',
+      csrfToken: '{{ csrf_token() }}',
+    });
+  }
+</script>
+{% endif %}
+{% endblock %}