Add /api/v1/rankings/assets endpoint: top assets by holders and volume

[christabel888]

Context: Grant-milestone-facing "asset rankings" endpoint, distinct from #33's 24h Top Movers -- this is a more general top-assets-by-holders-and-volume view.

Where:

• src/api/rankings.rs (same file as Add /api/v1/rankings/top-movers endpoint: 24h percent change by holders/volume #33)

What to do:

• Rank assets by absolute holder count and absolute volume (not percent change), for the frontend/mobile Asset Rankings page.

Suggested approach:

1. Same file as Add /api/v1/rankings/top-movers endpoint: 24h percent change by holders/volume #33 (src/api/rankings.rs), a second endpoint alongside top-movers.
2. This is mostly composition over data Add largest-holder-percent-of-supply computation per asset #7-Add most-active-counterparties aggregation: DEX vs payment address classification #10/Add /api/v1/rankings/top-movers endpoint: 24h percent change by holders/volume #33 already compute -- rank by absolute holders and absolute volume rather than percent change.

Watch out for:

• Decide whether this needs its own precompute/cache or can piggyback on Add /api/v1/rankings/top-movers endpoint: 24h percent change by holders/volume #33/Add hourly background job to compute top movers in src/jobs #34's existing precomputed data -- don't build a second parallel precompute pipeline if one already exists.

Definition of done:

• GET /api/v1/rankings/assets returns assets sorted by holders and by volume (two sort modes or two lists)