cli: demo experience

We want it to be easy for prospective users of Divvi Up to give it a whirl and do some simple aggregations. To enable that, we want to make it easy to use the included compose.yaml to spin up a control plane, web app and a pair of aggregators, and then use divviup dap-client to upload and collect.

Missing pieces are:

• Distribute compose.yaml in divviup-api releases so that users don't have to check out the project (#1098)
• Pairing aggregators with control plane in compose setup (#1099)
• rewrite cli/README.md to discuss docker compose (#1123)
• test in CI that docker compose up works (#1097)
• test demo script in CI (#1126)
• Automatically update container images in compose.yaml
• Do something about divviup_api_vite so you can run compose.yaml without checking out sources (#1124)
• fix startup race between postgres and migrator jobs (#1125)
• make collector credential handling less awkward (#1033, #1073)
• Publish the demo script (cli/README.md) somewhere like docs.divviup.org