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>

Dockerfile.transcoder

@@ -1,7 +1,7 @@
 # ==============================
 # Build stage
 # ==============================
-FROM node:20-alpine AS builder
+FROM node:22-alpine AS builder
 
 WORKDIR /app
 
@@ -25,7 +25,7 @@ RUN npm run build
 # ==============================
 # Runtime stage
 # ==============================
-FROM node:20-alpine
+FROM node:22-alpine
 
 # Install FFmpeg
 RUN apk add --no-cache ffmpeg
@@ -48,5 +48,8 @@ COPY prisma ./prisma
 ENV NODE_ENV=production
 ENV UPLOADS_DIR=/uploads
 
+# Run as the unprivileged "node" user (uid 1000) instead of root
+USER node
+
 # The container runs once and exits
 CMD ["node", "dist/index.js"]