Library P3 (hognek): heavy-tier media download - quality preference, storage accounting, per-source rules

[jaylfc]

Spec: docs/design/library-app.md section 3 heavy tier + section 4 step 5 (PR #2056). Deliver:

1. Opt-in download per item (yt-dlp format selection from a per-user preferred-quality setting + per-item override).
2. Storage accounting: bytes per item/source surfaced via the API; per-source cap that pauses further downloads at the cap (surfaced, never silent).
3. Per-source rules (e.g. 'this channel: always download at 720p').
4. Download progress via the jobs table; resumable/retryable.

Acceptance: trigger a download at a chosen quality, watch progress on the card, see the final size in accounting, hit a source cap and see the paused state.

[jaylfc]

Implementation notes for whoever picks this up: PR #2276 (merged) shipped the Library settings pane as the client half against a localStorage mock of this contract. Five review findings were deliberately deferred to this issue rather than blocking that scaffolding — see the review comment on #2276 (duplicate per-source rules allowed, storage-cap accepts 0/negative, literal-type state casts, saves inside setState updaters, and the end-user-visible "(mock until #2060)" label that must not survive this issue).

[hognek]

Claiming for @hognek / skald-engineer-taos. Library P3 (heavy-tier media download). Sequential on P2 (#2059).

[hognek]

Also already implemented and merged — #2070 (2026-08-27) "feat(library): P3 — heavy tier download, quality preferences, storage". This issue can be closed. Claim comment above was against a stale open-issue scan.