[db-sync] Install Redis into application cluster (or use GCP managed Redis)

[andrew-farries]

Talk with the workspace team regarding their experience of in-cluster Redis vs GCP MemoryStore.

[geropl]

Got feedback from them here. They had issues with node restarts and k3s not being able to re-join the cluster. But that's something we don't have to bother with. 👍

We should decide based on what's quicker, as we just want to invalidate some assumptions (in the context of db-sync removal, at least). A switch later on should always be doable.