Reverts the Dockerfile to the root src/ build. PR #47 switched to apps/api/
but the pnpm workspace config and lockfile don't include apps/*, causing
Docker build failures (CI runs #968, #970).
The actual admin 500 root cause needs further investigation — it's in the
code/schema changes, not the Dockerfile build path.
Co-Authored-By: Paperclip <noreply@paperclip.ing>