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/account_detail.html

@@ -15,6 +15,7 @@
     <dt class="text-muted">Status</dt>
     <dd id="status-cell">{{template "account_status" .}}</dd>
     <dt class="text-muted">TOTP</dt><dd>{{if .Account.TOTPRequired}}Enabled{{else}}Disabled{{end}}</dd>
+    {{if .WebAuthnEnabled}}<dt class="text-muted">Passkeys</dt><dd>{{len .WebAuthnCreds}} registered</dd>{{end}}
     <dt class="text-muted">Created</dt><dd class="text-small">{{formatTime .Account.CreatedAt}}</dd>
     <dt class="text-muted">Updated</dt><dd class="text-small">{{formatTime .Account.UpdatedAt}}</dd>
   </dl>
@@ -44,6 +45,12 @@
   <div id="token-delegates-section">{{template "token_delegates" .}}</div>
 </div>
 {{end}}
+{{if .WebAuthnEnabled}}
+<div class="card">
+  <h2 style="font-size:1rem;font-weight:600;margin-bottom:1rem">Passkeys</h2>
+  {{template "webauthn_credentials" .}}
+</div>
+{{end}}
 <div class="card">
   <h2 style="font-size:1rem;font-weight:600;margin-bottom:1rem">Tags</h2>
   <div id="tags-editor">{{template "tags_editor" .}}</div>