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.

web/embed.go

@@ -0,0 +1,17 @@
+// Package web embeds the templates and static assets of the administration
+// interface (ADR-0007). Handlers live in internal/ui.
+package web
+
+import (
+	"embed"
+	"io/fs"
+)
+
+//go:embed templates static
+var files embed.FS
+
+// Templates returns the template tree.
+func Templates() fs.FS { f, _ := fs.Sub(files, "templates"); return f }
+
+// Static returns the static asset tree.
+func Static() fs.FS { f, _ := fs.Sub(files, "static"); return f }