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/app/admin/users/page.tsx

@@ -0,0 +1,74 @@
+import Link from "next/link";
+import { prisma } from "@/lib/prisma";
+
+export const dynamic = "force-dynamic";
+
+const PER_PAGE = 25;
+
+export default async function AdminUsers({
+  searchParams,
+}: {
+  searchParams: Promise<{ q?: string; page?: string }>;
+}) {
+  const sp = await searchParams;
+  const q = (sp.q ?? "").trim();
+  const page = Math.max(1, Number(sp.page ?? "1") || 1);
+  const where = q ? { username: { contains: q } } : {};
+
+  const [users, total] = await Promise.all([
+    prisma.user.findMany({
+      where,
+      select: { id: true, username: true, rank: true, credits: true, online: true },
+      orderBy: { id: "desc" },
+      skip: (page - 1) * PER_PAGE,
+      take: PER_PAGE,
+    }),
+    prisma.user.count({ where }),
+  ]);
+  const pages = Math.max(1, Math.ceil(total / PER_PAGE));
+
+  return (
+    <main>
+      <h1>Users</h1>
+      <form style={{ display: "flex", gap: "0.5rem", marginBottom: "1rem" }}>
+        <input name="q" defaultValue={q} placeholder="Search username" />
+        <button type="submit">Search</button>
+      </form>
+
+      <table style={{ width: "100%", borderCollapse: "collapse" }}>
+        <thead>
+          <tr style={{ textAlign: "left", color: "var(--muted)" }}>
+            <th>ID</th>
+            <th>Username</th>
+            <th>Rank</th>
+            <th>Credits</th>
+            <th>Online</th>
+          </tr>
+        </thead>
+        <tbody>
+          {users.map((u) => (
+            <tr key={u.id} style={{ borderTop: "1px solid var(--border)" }}>
+              <td>{u.id}</td>
+              <td>
+                <Link href={`/admin/users/${u.id}`}>{u.username}</Link>
+              </td>
+              <td>{u.rank}</td>
+              <td>{u.credits}</td>
+              <td>{u.online === "1" ? "●" : "○"}</td>
+            </tr>
+          ))}
+        </tbody>
+      </table>
+
+      <p className="muted">
+        Page {page} / {pages} · {total} users
+        {page < pages ? (
+          <>
+            {" · "}
+            <Link href={`/admin/users?q=${encodeURIComponent(q)}&page=${page + 1}`}>Next →</Link>
+          </>
+        ) : null}
+      </p>
+    </main>
+  );
+}