feat: production hardening + smart subpage scanning with layout dedup

Security:
- Add CRON_SECRET auth to /api/cron/* endpoints
- Add admin role verification to /api/admin/* routes
- Add org membership check to /api/billing/usage
- Add security headers (HSTS, X-Frame-Options, CSP, etc.)
- Add env variable validation at startup
- Add rate limiting to backend API (30 req/min per IP)

Infrastructure:
- Multi-stage Dockerfiles with non-root user + healthchecks
- Updated cron workflow to pass CRON_SECRET header
- Updated .env.example with all optional vars

Smart subpage scanning:
- Crawler now computes template_hash (DOM structure without content)
- Scanner scans ALL unique-layout pages, not just main page
- Pages with same layout (e.g. product pages) scanned only once
- Deduplication by template_hash, fallback to content_hash
- Main page always scanned with high priority
- Re-checks subscription limits before each page scan

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

website-monitoring-backend/Dockerfile

@@ -1,25 +1,36 @@
-# Use the official Node.js image.
-FROM node:18
+# --- Stage 1: Build ---
+FROM node:20-slim AS builder
 
-# OPTIONAL: Falls in der Base kein Chrome enthalten ist,
-# müsstest du hier noch "apt-get update" + "apt-get install chromium" oder ähnliches ausführen,
-# z. B.:
-RUN apt-get update && apt-get install -y chromium
-
-# Create and change to the app directory.
 WORKDIR /app
 
-# Copy application dependency manifests to the container image.
 COPY package*.json ./
+RUN npm ci
 
-# Install production dependencies.
-RUN npm install
-
-# Copy local code to the container image.
 COPY . .
-
-# Build the TypeScript code
 RUN npm run build
 
-# Run the web service on container startup.
+# --- Stage 2: Production ---
+FROM node:20-slim AS runtime
+
+RUN apt-get update && apt-get install -y --no-install-recommends chromium \
+    && rm -rf /var/lib/apt/lists/*
+
+ENV CHROME_BIN=/usr/bin/chromium
+ENV PUPPETEER_SKIP_CHROMIUM_DOWNLOAD=true
+
+WORKDIR /app
+
+RUN groupadd -r app && useradd -r -g app -d /app app
+
+COPY --from=builder --chown=app:app /app/dist ./dist
+COPY --from=builder --chown=app:app /app/node_modules ./node_modules
+COPY --from=builder --chown=app:app /app/package.json ./
+
+USER app
+
+EXPOSE 5000
+
+HEALTHCHECK --interval=30s --timeout=5s --start-period=10s --retries=3 \
+  CMD node -e "const h=require('http');h.get('http://localhost:5000/health',(r)=>{process.exit(r.statusCode===200?0:1)}).on('error',()=>process.exit(1))"
+
 CMD ["node", "dist/index.js"]