Function compute_recency 

pub fn compute_recency(updated: &str, now_secs: u64) -> f64

Compute recency score (0.0–1.0) with exponential decay, half-life = 30 days.

Exposed so non-SQLite backends (e.g. the graph-pg PostgreSQL backend at src/graph/pg.rs) can score candidates with identical recency math — no drift across backends.