Add NFT activity API routes

[lai3d]

Summary

• add /tokens/nfts/transfers for recent NFT transfer activity
• add /tokens/nfts/mints for zero-address NFT mint activity
• keep response shape aligned with existing token transfer APIs for easy frontend consumption

Notes

• frontend updated separately to consume these dedicated routes
• local typecheck was not run here because this repo is missing local tsc in the current environment