Feat (dex): permanently store homeservers as graph nodes

[SHAcollision]

Super speculative and low priority task:

We currently have a Homserver model that we use to store pubky url and cursor for the homeserver in Redis. Storing them into the graph as well might be useful but there are some trade-offs. Note that cursor is expected to not be permanent: this way if we lose Redis data, we can sync again from the last cursor stored in Redis.

Maybe there is a use to know which user is hosted in what server when it comes to moderating spamming homeservers, etc. So optionally, maybe we can add a "HOSTED" relationship between User and Homeserver nodes. But: 1. we do not have a use for it 2. makes the graph larger / harder to traverse 3. will be hard to keep up to date as users migrate homeserver.