T55: Explainable decisions

[thedancingdeveloper]

Emit a structured, replayable object with the full per-specification score breakdown, exposed as an API resource. Sonarr's rejection reasons are strings in a log; 'why didn't it grab this?' is the most common complaint in the arr community.

A headline feature disguised as a refactor.

Acceptance: every grab/reject decision is queryable with its full reasoning.

[thedancingdeveloper]

🤖 Vogt validation pass — 2026-08-20 (automated review vs dd26a0aa)

Valid and still pending. The DownloadDecision struct (crates/stackarr-quality/src/lib.rs:428) records only a Vec of reason strings plus custom-format score; it captures no full per-specification pass/fail score breakdown, and decisions are computed inline in the /releases search path (routes/releases.rs) rather than persisted. There is no decisions API resource among the web routes and nothing makes a grab/reject decision replayable/queryable, so the acceptance is unmet.