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/fragments/webauthn_credentials.html

@@ -0,0 +1,30 @@
+{{define "webauthn_credentials"}}
+<div id="webauthn-credentials-section">
+{{if .WebAuthnCreds}}
+<table class="table" style="font-size:.85rem;margin-bottom:1rem">
+  <thead>
+    <tr><th>Name</th><th>Created</th><th>Last Used</th><th>Sign Count</th><th></th></tr>
+  </thead>
+  <tbody>
+  {{range .WebAuthnCreds}}
+    <tr>
+      <td>{{.Name}}{{if .Discoverable}} <span class="badge" title="Passkey (discoverable)">passkey</span>{{end}}</td>
+      <td class="text-small">{{formatTime .CreatedAt}}</td>
+      <td class="text-small">{{if .LastUsedAt}}{{formatTime (derefTime .LastUsedAt)}}{{else}}Never{{end}}</td>
+      <td>{{.SignCount}}</td>
+      <td>
+        <button class="btn btn-sm btn-danger"
+                hx-delete="{{$.DeletePrefix}}/{{.ID}}"
+                hx-target="#webauthn-credentials-section"
+                hx-swap="outerHTML"
+                hx-confirm="Remove this passkey?">Remove</button>
+      </td>
+    </tr>
+  {{end}}
+  </tbody>
+</table>
+{{else}}
+<p class="text-muted text-small">No passkeys registered.</p>
+{{end}}
+</div>
+{{end}}