[Performance] `get_reason_code_analytics` re-sorts full refund history on every call — result should be cached per window

[fati-Onchain]

Contract: contracts/refund/src/lib.rs

Description

Analytics are recomputed from the full refund dataset on every call. In a live system with thousands of refunds, this is a several-hundred-millisecond operation that fails at scale. The result is deterministic for a given ledger range and can be cached.

Fix

Cache the analytics result under DataKey::AnalyticsCache(window_start, window_end) → Vec<(reason_code, count)>. Invalidate the cache only when a new refund is processed within that window.