QBBB-FEAT-002 — Persist harvested torrents to a sharded local on-disk store

[itlezy]

Part of the post-0.7.3 suite forward program (Phase 1). Workflow state is tracked here; the spec lives in the linked doc.

Spec: https://github.com/emulebb/qbittorrentbb/blob/master/docs/active/items/QBBB-FEAT-002.md
Roadmap: emulebb-tooling docs/active/SUITE-JOINT-ROADMAP.md