docs(api): define v2 list and lookup contracts

[wklken]

Summary

• require separate list and exact batch lookup APIs under apis.v2
• standardize v2 list pagination on limit/offset and the count/results response envelope
• document the handling for already-paginated downstream sources

Verification

• git diff --check
• verified existing v2 -/lookup/ routes and standard pagination helpers with rg
• make lint/test not run: documentation-only change per repository guidance