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>

cmd/mciasdb/main.go

@@ -109,6 +109,8 @@ func main() {
 		tool.runAudit(subArgs)
 	case "pgcreds":
 		tool.runPGCreds(subArgs)
+	case "webauthn":
+		tool.runWebAuthn(subArgs)
 	case "rekey":
 		tool.runRekey(subArgs)
 	default:
@@ -245,6 +247,11 @@ Commands:
   account set-password   --id UUID  (prompts interactively)
   account set-status     --id UUID --status active|inactive|deleted
   account reset-totp     --id UUID
+  account reset-webauthn --id UUID
+
+  webauthn list          --id UUID
+  webauthn delete        --id UUID --credential-id N
+  webauthn reset         --id UUID
 
   role list              --id UUID
   role grant             --id UUID --role ROLE