Add new-holders classification for asset ranking cards

[christabel888]

Context: "New holders" is one of the two Top Movers card metrics (the other being percent volume/price change) -- needs the same holder-count-over-time data as #8.

Where:

• Same module as Add rolling 30-day holder-count growth computation per asset #8 (src/analytics/holders.rs)

What to do:

• Compute net-new holders in the last 24h per asset, for the Top Movers card's "X new holders" line.

Suggested approach:

1. Same module as Add rolling 30-day holder-count growth computation per asset #8 (src/analytics/holders.rs). Net-new holders in the last 24h = holders today minus holders 24h ago, using the same snapshot infra as Add rolling 30-day holder-count growth computation per asset #8.

Watch out for:

• "New holder" vs "holder who briefly held then left" can look the same in a single before/after snapshot -- if that distinction matters for the product narrative (it does, per the pivot doc's "X new holders" framing), you may need more than 2 snapshot points.

Definition of done:

• Value included in Add /api/v1/rankings/top-movers endpoint: 24h percent change by holders/volume #33's ranking response