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>

ARCHITECTURE.md

@@ -128,7 +128,8 @@ mciassrv (passphrase or keyfile) to decrypt secrets at rest.
 **Human accounts** — interactive users. Can authenticate via:
 - Username + password (Argon2id hash stored in DB)
 - Optional TOTP (RFC 6238); if enrolled, required on every login
-- Future: FIDO2/WebAuthn, Yubikey (not in scope for v1)
+- Optional FIDO2/WebAuthn passkeys and security keys; discoverable credentials
+  enable passwordless login, non-discoverable credentials serve as 2FA
 
 **System accounts** — non-interactive service identities. Have:
 - A single active bearer token at a time (rotating the token revokes the old one)
@@ -420,6 +421,17 @@ value in an HTMX fragment or flash message.
 | POST | `/v1/auth/totp/confirm` | bearer JWT | Confirm TOTP enrollment with code |
 | DELETE | `/v1/auth/totp` | admin JWT | Remove TOTP from account (admin) |
 
+### WebAuthn Endpoints
+
+| Method | Path | Auth required | Description |
+|---|---|---|---|
+| POST | `/v1/auth/webauthn/register/begin` | bearer JWT | Begin WebAuthn registration (requires password re-auth) |
+| POST | `/v1/auth/webauthn/register/finish` | bearer JWT | Complete WebAuthn registration |
+| POST | `/v1/auth/webauthn/login/begin` | none | Begin WebAuthn login (discoverable or username-scoped) |
+| POST | `/v1/auth/webauthn/login/finish` | none | Complete WebAuthn login, returns JWT |
+| GET | `/v1/accounts/{id}/webauthn` | admin JWT | List WebAuthn credential metadata |
+| DELETE | `/v1/accounts/{id}/webauthn/{credentialId}` | admin JWT | Remove WebAuthn credential |
+
 ### Postgres Credential Endpoints
 
 | Method | Path | Auth required | Description |
@@ -697,14 +709,36 @@ CREATE INDEX idx_sa_delegates_account ON service_account_delegates (account_id);
 CREATE INDEX idx_sa_delegates_grantee ON service_account_delegates (grantee_id);
 ```
 
+```sql
+-- WebAuthn credentials (migration 000009)
+CREATE TABLE webauthn_credentials (
+    id                  INTEGER PRIMARY KEY,
+    account_id          INTEGER NOT NULL REFERENCES accounts(id) ON DELETE CASCADE,
+    name                TEXT    NOT NULL DEFAULT '',
+    credential_id_enc   BLOB    NOT NULL,
+    credential_id_nonce BLOB    NOT NULL,
+    public_key_enc      BLOB    NOT NULL,
+    public_key_nonce    BLOB    NOT NULL,
+    aaguid              TEXT    NOT NULL DEFAULT '',
+    sign_count          INTEGER NOT NULL DEFAULT 0,
+    discoverable        INTEGER NOT NULL DEFAULT 0,
+    transports          TEXT    NOT NULL DEFAULT '',
+    created_at          TEXT    NOT NULL,
+    updated_at          TEXT    NOT NULL,
+    last_used_at        TEXT
+);
+CREATE INDEX idx_webauthn_credentials_account ON webauthn_credentials(account_id);
+```
+
 ### Schema Notes
 
 - Passwords are stored as PHC-format Argon2id strings (e.g.,
   `$argon2id$v=19$m=65536,t=3,p=4$<salt>$<hash>`), embedding algorithm
   parameters. Future parameter upgrades are transparent.
-- TOTP secrets and Postgres passwords are encrypted with AES-256-GCM using a
-  master key held only in server memory (derived at startup from a passphrase
-  or keyfile). The nonce is stored adjacent to the ciphertext.
+- TOTP secrets, Postgres passwords, and WebAuthn credential IDs/public keys are
+  encrypted with AES-256-GCM using a master key held only in server memory
+  (derived at startup from a passphrase or keyfile). The nonce is stored
+  adjacent to the ciphertext.
 - The master key salt is stored in `server_config.master_key_salt` so the
   Argon2id KDF produces the same key on every restart. Generated on first run.
 - The signing key encryption is layered: the Ed25519 private key is wrapped
@@ -782,7 +816,7 @@ mcias/
 │   ├── config/         # config file parsing and validation
 │   ├── crypto/         # key management, AES-GCM helpers, master key derivation
 │   ├── db/             # SQLite access layer (schema, migrations, queries)
-│   │   └── migrations/ # numbered SQL migrations (currently 8)
+│   │   └── migrations/ # numbered SQL migrations (currently 9)
 │   ├── grpcserver/     # gRPC handler implementations
 │   ├── middleware/     # HTTP middleware (auth extraction, logging, rate-limit, policy)
 │   ├── model/          # shared data types (Account, Token, Role, PolicyRule, etc.)
@@ -791,7 +825,8 @@ mcias/
 │   ├── token/          # JWT issuance, validation, revocation
 │   ├── ui/             # web UI context, CSRF, session, template handlers
 │   ├── validate/       # input validation helpers (username, password strength)
-│   └── vault/          # master key lifecycle: seal/unseal state, key derivation
+│   ├── vault/          # master key lifecycle: seal/unseal state, key derivation
+│   └── webauthn/       # FIDO2/WebAuthn adapter (encrypt/decrypt credentials, user interface)
 ├── web/
 │   ├── static/         # CSS, JS, and bundled swagger-ui assets (embedded at build)
 │   ├── templates/      # HTML templates (base layout, pages, HTMX fragments)
@@ -1830,3 +1865,66 @@ the "Token Issue Access" section.
 | `token_delegate_granted` | Admin granted a human account token-issue access for a system account |
 | `token_delegate_revoked` | Admin revoked token-issue delegation |
 | `token_issued` | Token issued (existing event, also fires for delegate-issued tokens) |
+
+## 22. FIDO2/WebAuthn Authentication
+
+### Overview
+
+WebAuthn support enables two credential modes:
+- **Discoverable credentials (passkeys)** — passwordless login. The authenticator
+  stores a resident credential; the user clicks "Sign in with passkey" and the
+  browser prompts for the credential directly.
+- **Non-discoverable credentials (security keys)** — 2FA alongside
+  username+password. The server supplies allowCredentials for the account.
+
+Either WebAuthn or TOTP satisfies the 2FA requirement. If both are enrolled the
+UI offers passkey first.
+
+### Credential Storage
+
+Credential IDs and public keys are encrypted at rest with AES-256-GCM using
+the vault master key, consistent with TOTP secrets and PG credentials. The
+nonce is stored alongside the ciphertext in the `webauthn_credentials` table.
+
+Metadata (name, AAGUID, sign count, discoverable flag, transports, timestamps)
+is stored in plaintext for display and management.
+
+### Challenge (Ceremony) Management
+
+Registration and login ceremonies use an in-memory `sync.Map` with 120-second
+TTL, consistent with the `pendingLogins` and `tokenDownloads` patterns. Each
+ceremony is keyed by a 128-bit random nonce. Ceremonies are single-use:
+consumed on finish, expired entries cleaned by a background goroutine.
+
+Separate ceremony stores exist for REST API (`internal/server`) and web UI
+(`internal/ui`) to maintain independent lifecycle management.
+
+### Sign Counter Validation
+
+On each assertion the stored sign counter is compared to the authenticator's
+reported value. If the reported counter is less than or equal to the stored
+counter (and both are non-zero), the assertion is rejected as a potential
+cloned authenticator. This mirrors the TOTP replay protection pattern.
+
+### Audit Events
+
+| Event | Description |
+|---|---|
+| `webauthn_enrolled` | New WebAuthn credential registered |
+| `webauthn_removed` | WebAuthn credential removed (self-service or admin) |
+| `webauthn_login_ok` | Successful WebAuthn authentication |
+| `webauthn_login_fail` | Failed WebAuthn authentication attempt |
+
+### Configuration
+
+WebAuthn is enabled by adding a `[webauthn]` section to the TOML config:
+
+```toml
+[webauthn]
+rp_id = "mcias.metacircular.net"
+rp_origin = "https://mcias.metacircular.net:8443"
+display_name = "MCIAS"
+```
+
+If the section is omitted, WebAuthn endpoints return 404 and the UI hides
+passkey-related controls.