Add FIDO2/WebAuthn passkey authentication

Phase 14: Full WebAuthn support for passwordless passkey login and
hardware security key 2FA.

- go-webauthn/webauthn v0.16.1 dependency
- WebAuthnConfig with RPID/RPOrigin/DisplayName validation
- Migration 000009: webauthn_credentials table
- DB CRUD with ownership checks and admin operations
- internal/webauthn adapter: encrypt/decrypt at rest with AES-256-GCM
- REST: register begin/finish, login begin/finish, list, delete
- Web UI: profile enrollment, login passkey button, admin management
- gRPC: ListWebAuthnCredentials, RemoveWebAuthnCredential RPCs
- mciasdb: webauthn list/delete/reset subcommands
- OpenAPI: 6 new endpoints, WebAuthnCredentialInfo schema
- Policy: self-service enrollment rule, admin remove via wildcard
- Tests: DB CRUD, adapter round-trip, interface compliance
- Docs: ARCHITECTURE.md §22, PROJECT_PLAN.md Phase 14

Security: Credential IDs and public keys encrypted at rest with
AES-256-GCM via vault master key. Challenge ceremonies use 128-bit
nonces with 120s TTL in sync.Map. Sign counter validated on each
assertion to detect cloned authenticators. Password re-auth required
for registration (SEC-01 pattern). No credential material in API
responses or logs.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

web/templates/login.html

@@ -29,10 +29,24 @@
           <button class="btn btn-primary" type="submit" style="width:100%">Sign in</button>
         </div>
       </form>
+      {{if .WebAuthnEnabled}}
+      <div style="margin-top:1rem;text-align:center">
+        <div style="display:flex;align-items:center;gap:.75rem;margin-bottom:.75rem">
+          <hr style="flex:1;border:0;border-top:1px solid #ddd">
+          <span class="text-muted text-small">or</span>
+          <hr style="flex:1;border:0;border-top:1px solid #ddd">
+        </div>
+        <div id="webauthn-login-error" class="alert alert-error" style="display:none" role="alert"></div>
+        <button class="btn btn-secondary" type="button" id="webauthn-login-btn" style="width:100%">
+          Sign in with passkey
+        </button>
+      </div>
+      {{end}}
     </div>
   </div>
 </div>
 <script src="/static/htmx.min.js"></script>
+{{if .WebAuthnEnabled}}<script src="/static/webauthn.js"></script>{{end}}
 </body>
 </html>
 {{end}}