Reuse slide translations across services, not only saved ones #187

Problem

The library (slideLibrary.ts) only holds what a reviewer explicitly saved. The review screen's Save is its only writer. Everything else lives only in the per-day doc and is gone next week:

• translate-ahead drafts;
• agent follow-up revisions (applyUpdates in SlideReviewContainer.tsx);
• the model's per-slide notes (Surface the translator's per-slide notes on the entry #184).

So a hymn sung every month is drafted from scratch, and paid for, every time unless someone pressed Save. Each new draft is as likely to make a good translation worse as to make a bad one better. Policy: reuse what was translated before; re-translate only when someone explicitly asks.

Design questions

• Where it lives. Either the library caches every draft, or a separate draft store sits beside it. provenance (llm vs human) already records which entries were human-saved. That is a fact about the text's origin, not the reviewed/auto status flag Drop the reviewed/auto tier from slide translations #182 removed.
• The Saved chip (SlideReview.tsx) compares the draft to the library. If drafts land in the library too, every draft reads "Saved". The chip would have to key on provenance: 'human' instead.
• Notes ride along. If a draft is reused next week without its note, the reason it needed a look is lost.
• What an explicit Re-translate writes. Today it fills the review grid only (in the browser). Viewers see it only after a per-cell Save, and the picker's note count doesn't see it. If explicit is the only way to re-translate, its result should probably reach the store directly.
• What happens to a bad cached draft. Without a human looking at it, it sticks. That is the accepted cost of the policy, but the review screen should make a stale or poor draft easy to find. Notes are part of that.

Relationship

Makes the doc lookup in #186 redundant once in place; #186 is the cheap stopgap. Overlaps with the conversation issue, because a reused translation's history (prior feedback) is keyed differently from the translation itself.