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/webauthn/adapter.go

@@ -0,0 +1,28 @@
+// Package webauthn provides the adapter between the go-webauthn library and
+// MCIAS internal types. It handles WebAuthn instance configuration and
+// encryption/decryption of credential material stored in the database.
+package webauthn
+
+import (
+	"fmt"
+
+	"github.com/go-webauthn/webauthn/webauthn"
+
+	"git.wntrmute.dev/kyle/mcias/internal/config"
+)
+
+// NewWebAuthn creates a configured go-webauthn instance from MCIAS config.
+func NewWebAuthn(cfg *config.WebAuthnConfig) (*webauthn.WebAuthn, error) {
+	if cfg.RPID == "" || cfg.RPOrigin == "" {
+		return nil, fmt.Errorf("webauthn: RPID and RPOrigin are required")
+	}
+	displayName := cfg.DisplayName
+	if displayName == "" {
+		displayName = "MCIAS"
+	}
+	return webauthn.New(&webauthn.Config{
+		RPID:          cfg.RPID,
+		RPDisplayName: displayName,
+		RPOrigins:     []string{cfg.RPOrigin},
+	})
+}