Accurate per-item liquidity and estimated sale time

GET /v1/liquidity/items returns a liquidity bucket and an estimated sale time for every item, recomputed daily from real price and sale-volume history on BUFF, Steam, and CSFloat:

{
  "liquidity": "very_liquid",
  "estimated_sale_time": "hours - same day"
}

Variants are scored separately, so Doppler phases and Case Hardened tiers each get their own liquidity rather than inheriting the base item's.

This replaces the older /v1/liquidity rank endpoint, which has been removed. See /v1/liquidity/items for the full list of buckets.