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>

static/css/marketing.css

@@ -2477,6 +2477,9 @@
   .text-brand-navy {
     color: #060d1a;
   }
+  .text-brand-navy\/50 {
+    color: color-mix(in oklab, #060d1a 50%, transparent);
+  }
   .text-brand-navy\/60 {
     color: color-mix(in oklab, #060d1a 60%, transparent);
   }
@@ -3628,6 +3631,13 @@
       }
     }
   }
+  .hover\:text-red-900 {
+    &:hover {
+      @media (hover: hover) {
+        color: var(--color-red-900);
+      }
+    }
+  }
   .hover\:text-white {
     &:hover {
       @media (hover: hover) {
@@ -3790,6 +3800,11 @@
       outline-color: #0062ff;
     }
   }
+  .focus-visible\:outline-red-700 {
+    &:focus-visible {
+      outline-color: var(--color-red-700);
+    }
+  }
   .active\:scale-95 {
     &:active {
       --tw-scale-x: 95%;