feat(auth): two-role permission system (admin/member)

Adds an admin/member role to WebUI auth. /api/users and /api/audit
are now gated by a requireAdmin middleware; all other authenticated
endpoints accept both roles. Schema migration defaults all existing
users to admin to preserve access. POST /api/users defaults new users
to member; first-run setup always creates an admin. Adds PATCH
/api/users/:id/role with last-admin demotion and deletion guards.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/web/middleware/requireAdmin.ts

@@ -0,0 +1,13 @@
+import type { Request, Response, NextFunction } from "express";
+
+export function requireAdmin(req: Request, res: Response, next: NextFunction): void {
+  if (!req.user) {
+    res.status(401).json({ error: "unauthenticated" });
+    return;
+  }
+  if (req.user.role !== "admin") {
+    res.status(403).json({ error: "forbidden" });
+    return;
+  }
+  next();
+}