Add docker prod environment

[chetanr25]

Add docker/prod/ environment (multi-stage, no mount, gunicorn)

Context

Companion to the dev environment issue. This covers the prod image: small, reproducible, no source mount.

Goal

docker/
  prod/
    Dockerfile     # multi-stage build, runtime deps only
    compose.yml    # app behind gunicorn (+ workers), ollama, nginx hookup

Tasks

• docker/prod/Dockerfile:
  • multi-stage: build stage installs/builds, runtime stage copies only what's needed
  • runtime deps only (no dev tools)
  • no source bind-mount; code is baked into the image
  • run via gunicorn with uvicorn workers against backend.main
• docker/prod/compose.yml:
  • app + ollama
  • reference conf/nginx/ for the reverse proxy (config files tracked separately)
  • production-sane restart policies
• Update command in Makefile later

Part of #512