Patch React2Shell RCE and harden auth, uploads and containers

- Upgrade next 16.0.3 -> 16.3.5 and react/react-dom 19.2.0 -> 19.3.0
  (CVE-2025-55182 / CVE-2025-66478 unauthenticated RCE), next-auth -> 4.24.15
- Require admin role on /api/users, /api/courses and /api/enrollments,
  which were unauthenticated
- Replace ALLOWED_ADMINS checks that allowed any signed-in user when the
  variable was empty with the role-based requireAdmin
- Sanitize upload filenames, restrict thumbnail extensions, store edited
  thumbnails in UPLOADS_DIR, tighten traversal checks in file routes
- Dockerfile: Node 22, pinned pnpm, non-root user, critical audit gate
- Compose: read-only rootfs, dropped caps, no-new-privileges, Postgres
  password from env
- Exclude .env from Docker build context; add security headers
- Deploy with git reset --hard instead of git pull
- Remove stale package-lock.json

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

app/api/users/route.ts

@@ -1,9 +1,13 @@
 // app/api/users/route.ts
 import { NextResponse } from "next/server";
 import { prisma } from "../../../lib/prisma";
+import { adminGuard } from "@/lib/admin-check";
 
 
 export async function GET() {
+  const denied = await adminGuard();
+  if (denied) return denied;
+
   const users = await prisma.user.findMany({ select: { id: true, name: true, email: true } });
   return NextResponse.json(users);
 }