Remove @entity annotation from enum types #177

As pointed out by @jmulq, there are several enum types in our schema annotated with @entity:

According to The Graph docs, enum types cannot be entities:

• https://thegraph.com/docs/en/subgraphs/guides/enums/#what-are-enums

Although the subgraph currently deploys, this is not correct, so we should remove the @entity annotation from these enum definitions.

Goal: align the schema with The Graph’s specifications, avoid relying on undefined behavior, and prevent potential issues in future deployments or tooling.