Function usage_dedup_key 

pub fn usage_dedup_key(hash_tag: &str, dedup_id: &str) -> String

Build a usage-dedup key: ff:usagededup:<hash_tag>:<dedup_id>.

hash_tag must ALREADY include the Valkey hash-tag braces (e.g. "{bp:7}") — typically obtained from BudgetKeyContext::hash_tag. Do not double-wrap.