Add Dockerfile, frontend image, and local compose stack

Dockerfile

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+# Shattered Void MMO - backend (Express + WebSocket)
+# bookworm-slim rather than alpine: bcrypt ships glibc prebuilds, musl would force a source build.
+
+FROM node:20-bookworm-slim AS deps
+WORKDIR /app
+COPY package.json package-lock.json ./
+RUN npm ci
+
+FROM node:20-bookworm-slim AS runtime
+WORKDIR /app
+
+RUN apt-get update \
+    && apt-get install -y --no-install-recommends curl \
+    && rm -rf /var/lib/apt/lists/*
+
+COPY --from=deps /app/node_modules ./node_modules
+COPY . .
+
+# winston-daily-rotate-file writes here but does not create the directory itself
+RUN mkdir -p logs && chown -R node:node /app
+
+USER node
+EXPOSE 3000
+
+HEALTHCHECK --interval=15s --timeout=5s --start-period=30s --retries=5 \
+    CMD curl -fsS http://localhost:3000/health || exit 1
+
+CMD ["node", "src/server.js"]