SPIKE: Install and configure Pants for builds/deployments

**Describe the task**
In order to:

a) build and deploy only apps that are changed
b) isolate risky infra changes to specific apps
c) maintain all code in a single codebase (monorepo)

We can install and configure Pants to do this: https://www.pantsbuild.org

This will help us make quicker changes with more confidence that changes won't break

**Acceptance Criteria**
- [ ] Know if Pants is sufficient for isolating app builds/deployments

**Additional context**
- https://www.pantsbuild.org/docs/using-pants-in-ci
- https://blog.pantsbuild.org/effective-monorepos-with-pants/
- https://semaphoreci.com/blog/building-python-projects-with-pants#h-pants-change-detection
- https://blog.pantsbuild.org/pants-pex-and-docker/

Start by reviewing the linked Pants CI, monorepo, change-detection, and Pants/PEX/Docker resources, then inspect this repository's current build and deployment setup. Evaluate whether Pants can isolate builds and deployments for changed applications and risky infrastructure changes. Done means documenting whether Pants is sufficient for the stated goals.

Written by the indexing model from the issue text.