chore: two Dockerfiles (root + backend/) can silently diverge

[singret]

Problem

There are two Dockerfiles in the repo:

• Dockerfile (root) — used by docker-compose.yml (context: ., dockerfile: Dockerfile)
• backend/Dockerfile — appears to be a leftover from an earlier layout

A developer editing backend/Dockerfile thinking they are updating the production image will silently change nothing.

How we found it

During a production readiness checklist run we fixed the root Dockerfile's HEALTHCHECK but accidentally edited backend/Dockerfile first. The build silently used the old HEALTHCHECK for two rebuild cycles before we noticed the wrong file was being changed.

Risk

• Silent no-ops for contributors editing the wrong file
• HEALTHCHECK, EXPOSE, labels, and other metadata can drift
• CI builds the root Dockerfile; contributors may accidentally test backend/Dockerfile

Proposed fix

Delete backend/Dockerfile — it serves no purpose now that the build context is the repo root.