Calculate rarity score per ERC-721 items

for ERC-721 add

```graphql
rarity: Float!
```

after create was called:
- check if you indexed max
- calculate rarity on last fetched item

ref
- https://github.com/kodadot/nft-gallery/issues/3181

articles:
- [How to Calculate the Rarity of an NFT](https://hackernoon.com/how-to-calculate-the-rarity-of-an-nft)
- [Understanding Rarity Calculation Methods](https://raritytools.medium.com/ranking-rarity-understanding-rarity-calculation-methods-86ceaeb9b98c)

Start by locating the ERC-721 GraphQL type and the create/index flow described in the issue, then review the linked reference and rarity-calculation articles. Done means the schema exposes a required Float rarity field and the indexed items receive calculated rarity values after creation, including the max-index check.

Written by the indexing model from the issue text.