Build production image excluding devDependencies

[robertocarlous]

Body:

Dev-only packages bloat production images and may introduce vulnerabilities. Update the Dockerfile to use multi-stage builds or npm ci --production so devDependencies are not present in the final runtime image.

Acceptance Criteria:

• Dockerfile uses multi-stage build or otherwise excludes devDependencies.
• Final image size is reduced compared to the previous baseline.
• Build pipeline runs and produces an image that starts and serves the /health endpoint.