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>

internal/policy/defaults.go

@@ -81,6 +81,16 @@ var defaultRules = []Rule{
 		OwnerMatchesSubject: true,
 		Effect:              Allow,
 	},
+	{
+		// Self-service WebAuthn enrollment: any authenticated human account may
+		// register and manage their own passkeys/security keys. The handler
+		// verifies the subject matches before writing. Mirrors TOTP rule -3.
+		ID:          -8,
+		Description: "Self-service: any principal may enroll their own WebAuthn credentials",
+		Priority:    0,
+		Actions:     []Action{ActionEnrollWebAuthn},
+		Effect:      Allow,
+	},
 	{
 		// Public endpoints: token validation and login do not require
 		// authentication. The middleware exempts them from RequireAuth entirely;