Add admin foundation + Users resource (Filament replacement, slice 1)

Plain App Router admin (aligned to habbo-next, no Refine):
- rank surfaced on the NextAuth session; staff guard isStaff() [pure,
  unit-tested] + requireStaff() reading min_staff_rank, gating /admin.
- /admin dashboard (counts), /admin/users (paginated + search),
  /admin/users/[id] detail.
- src/actions/admin-users.ts: staff-gated server actions wiring the user editor
  to the existing services — giveCurrency (RCON or DB fallback), setMotto/setRank
  (DB + RCON), alertUser, disconnectUser.

Verified: tsc exit 0, vitest 45/45, next build exit 0 (/admin routes).

src/lib/admin/guard.ts

@@ -0,0 +1,33 @@
+import { redirect } from "next/navigation";
+import { isStaff } from "@/lib/admin/is-staff";
+import { auth } from "@/lib/auth";
+import { siteSettings } from "@/lib/services/site-settings";
+
+export { isStaff };
+
+export async function getMinStaffRank(): Promise<number> {
+  const n = Number(await siteSettings.get("min_staff_rank", "7"));
+  return Number.isFinite(n) ? n : 7;
+}
+
+export interface StaffUser {
+  id: number;
+  rank: number;
+  username: string;
+}
+
+/**
+ * Gate for admin pages and actions: redirects to /login when unauthenticated
+ * and to / when authenticated but not staff. Returns the staff user otherwise.
+ */
+export async function requireStaff(): Promise<StaffUser> {
+  const session = await auth();
+  if (!session?.user?.id) redirect("/login");
+  const minRank = await getMinStaffRank();
+  if (!isStaff(session.user.rank, minRank)) redirect("/");
+  return {
+    id: Number(session.user.id),
+    rank: session.user.rank,
+    username: session.user.name ?? "",
+  };
+}

src/lib/admin/is-staff.test.ts

@@ -0,0 +1,14 @@
+import { describe, expect, it } from "vitest";
+import { isStaff } from "./is-staff";
+
+describe("isStaff", () => {
+  it("is true at or above the min staff rank", () => {
+    expect(isStaff(7, 7)).toBe(true);
+    expect(isStaff(10, 7)).toBe(true);
+  });
+
+  it("is false below the min staff rank", () => {
+    expect(isStaff(6, 7)).toBe(false);
+    expect(isStaff(1, 7)).toBe(false);
+  });
+});

src/lib/admin/is-staff.ts

@@ -0,0 +1,4 @@
+/** AtomCMS housekeeping gate: staff are users with rank >= min_staff_rank. */
+export function isStaff(rank: number, minStaffRank: number): boolean {
+  return rank >= minStaffRank;
+}