T59: Unified download queue and resource budgets

[thedancingdeveloper]

One queue, one disk budget, one bandwidth budget. Today the arrs fight each other for client slots and free space.

Acceptance: a global priority ordering across TV and film; disk reservations honoured across media types.

[thedancingdeveloper]

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

Valid and pending: T59 is unbuilt on origin/main. crates/stackarr-download holds only per-client managers (manager.rs, qbittorrent.rs, sabnzbd.rs, nzbget.rs, embedded_torrent.rs) with no unified queue — a repo-wide grep for disk/bandwidth/resource budget, reservation, and global priority ordering returns nothing. The acceptance criteria (global priority across TV/film, disk reservations across media types) are not met. This phase:p7 roadmap task remains valid and pending.