Add a Django DB migration script compatible with GitOps

This is simple in principle, but is complicated by the requirement of rolling back by running a historic pipeline. What's needed is a data snapshot coupled to the Git SHA, and ensure that it's deployed together.

A possible solution:

•Deployment Repo
→ incoming commit of tfvars plus the new Git SHA (of the application build commit)
  → pipeline triggered
    → job checks for previous data snapshot tagged with the new Git SHA
        true:  write pipeline stage to restore that snapshot
        false: write pipeline stage to take a snapshot tagged with the existing Git SHA
    → job Terraform plan
    → job Terraform apply
        run that pipeline stage
        run terraform apply

... then the migration job can follow