feat: add authentication, audit log, HTTP API and base web UI

Phase 1 foundation. Local auth per ADR-0006: Argon2id passwords, opaque
hashed session tokens with idle/absolute expiry, CSRF token plus Origin
check on cookie sessions, per-IP token bucket and per-account exponential
lockout, three roles enforced by middleware, personal API tokens for
noemactl. Audit log writer used by every admin mutation.

API: request ids, panic recovery, strict CSP and security headers, body
limits, single JSON error envelope, health/ready/status, users, sessions,
tokens, audit, token-protected /metrics.

UI: embedded html/template pages (login, first-run setup, dashboard,
system, users, account, audit, interpretation boundary), light/dark
theme, dependency-free app.js with CSRF-aware fetch and SSE helper.

Binaries: noemad with graceful shutdown, migrations at start, bootstrap
admin from environment, supervised background tasks; noemactl with
status, migrate, user create and meaningful exit codes.

internal/api/errors_util.go

@@ -0,0 +1,11 @@
+package api
+
+import (
+	"errors"
+
+	"github.com/andydixon/noema/internal/storage"
+)
+
+func isStorageErr(err error) bool {
+	return errors.Is(err, storage.ErrNotFound) || errors.Is(err, storage.ErrConflict)
+}