feat: add Docker containerization (Dockerfile, compose, dockerignore)

Multi-stage build: frontend (Vite) + backend (TypeScript) in one container.
Production image based on node:20-alpine with health check.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Dockerfile

@@ -0,0 +1,53 @@
+# ============================================================
+# Stage 1: Build Frontend (React + Vite)
+# ============================================================
+FROM node:20-alpine AS build-frontend
+
+WORKDIR /app/frontend
+COPY frontend/package.json frontend/package-lock.json ./
+RUN npm ci
+COPY frontend/ ./
+RUN npm run build
+# Output: /app/frontend/../backend/public → /app/backend/public
+
+# ============================================================
+# Stage 2: Build Backend (TypeScript → JavaScript)
+# ============================================================
+FROM node:20-alpine AS build-backend
+
+WORKDIR /app/backend
+COPY backend/package.json backend/package-lock.json ./
+RUN npm ci
+COPY backend/ ./
+RUN npx tsc
+
+# ============================================================
+# Stage 3: Production Image
+# ============================================================
+FROM node:20-alpine AS production
+
+WORKDIR /app
+
+# Nur Production-Dependencies installieren
+COPY backend/package.json backend/package-lock.json ./
+RUN npm ci --omit=dev && npm cache clean --force
+
+# Compiled Backend
+COPY --from=build-backend /app/backend/dist ./dist
+
+# Frontend Build (statische Dateien)
+COPY --from=build-frontend /app/backend/public ./public
+
+# Non-root User
+RUN addgroup -S appgroup && adduser -S appuser -G appgroup
+USER appuser
+
+ENV NODE_ENV=production
+ENV PORT=3001
+
+EXPOSE 3001
+
+HEALTHCHECK --interval=30s --timeout=5s --start-period=10s --retries=3 \
+  CMD wget -qO- http://localhost:3001/health || exit 1
+
+CMD ["node", "dist/index.js"]