refactor: flatten monorepo structure to backend/ frontend/ devops/

Rename subdirectories for a cleaner single-repo layout:
- website-monitoring-backend/  → backend/
- website-monitoring-frontend/ → frontend/
- website-monitoring-devops/   → devops/

Update all references in package.json scripts, CI workflows,
docker-compose, pre-commit hooks, and documentation.

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

backend/Dockerfile

@@ -0,0 +1,36 @@
+# --- Stage 1: Build ---
+FROM node:20-slim AS builder
+
+WORKDIR /app
+
+COPY package*.json ./
+RUN npm ci
+
+COPY . .
+RUN npm run build
+
+# --- 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"]