fix: add Docker native HEALTHCHECK, replaces broken Coolify HTTP check

Coolify's HTTP health check hits localhost:3000 from the host — but the
container port is only reachable inside the Docker network via Traefik.
Docker's own HEALTHCHECK runs inside the container where localhost works.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Dockerfile

@@ -1,5 +1,7 @@
 FROM node:20-alpine
 
+RUN apk add --no-cache curl
+
 WORKDIR /app
 
 COPY package*.json ./
@@ -9,4 +11,7 @@ COPY src/ ./src/
 
 EXPOSE 3000
 
+HEALTHCHECK --interval=15s --timeout=5s --start-period=30s --retries=3 \
+  CMD curl -f http://localhost:3000/health || exit 1
+
 CMD ["node", "src/index.js"]