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/profile.html

@@ -4,6 +4,18 @@
 <div class="page-header">
   <h1>Profile</h1>
 </div>
+{{if .WebAuthnEnabled}}
+<div class="card">
+  <h2 style="font-size:1rem;font-weight:600;margin-bottom:1rem">Passkeys</h2>
+  <p class="text-muted text-small" style="margin-bottom:.75rem">
+    Passkeys let you sign in without a password using your device's biometrics or a security key.
+  </p>
+  {{template "webauthn_credentials" .}}
+  <h3 style="font-size:.9rem;font-weight:600;margin:1rem 0 .5rem">Add a Passkey</h3>
+  {{template "webauthn_enroll" .}}
+</div>
+<script src="/static/webauthn.js"></script>
+{{end}}
 <div class="card">
   <h2 style="font-size:1rem;font-weight:600;margin-bottom:1rem">Change Password</h2>
   <p class="text-muted text-small" style="margin-bottom:.75rem">