Metaforecast move to this repo

[berekuk]

We've agreed to move Metaforecast repo to this monorepo.

Benefits:

• common setup such as typescript configs, CI pipeline, etc.st
• ability to reuse Metaforecast code (DB library that I plan to extract to a separate package) in other code, i.e. Squiggle AI runs and
• ability to reuse @quri/ui and other code from Metaforecast

[berekuk]

One complication is CI/CD:

• Metaforecast backend deploys to our k8s cluster
• the old GitHub Action used the GitOps pattern where the image tag is committed back to the repo
• I don't want to do extra commits to the Squiggle repo though (it's possible but creates many fake commits which I don't like)

For this, I'm investigating Argo Image Updater on the ops side (so this is more of a task for https://github.com/quantified-uncertainty/ops repo).

My current plan:

• keep Helm chart for Metaforecast in this repo (old metaforecast repo already had a Dockerfile)
• build Docker image in CI for this repo; use turbo-ignore so that we don't do it too often
• don't provide image.tag in the chart's values.yaml; provide it from the ops repo side
• configure Argo Image Updater with git write-back method to commit image.tag to ops repo when new image is detected

If this patterns works well, then I hope we'll be able to use it for other CI/CD to k8s pipelines that we have, for example for guesstimate backend and for squiggle background builds.