feat(proxy): reclaim space from the blob store

Clearing `incoming` unlinks partials that live fetches and `link` are using

Defect 17 / Tier 4 / blocking

services/proxy/src/sweep.rs:219-236

Expected review

only remove a partial older than some generous threshold - longer than `CAIRN_FETCH_TIMEOUT` - so no write in progress can be one of them.

Discovery difficulty

the doc comment argues that a partial needs no age check, and it is nearly right: a partial carries no digest and nothing can look one up. The step it misses is that the process holding it open is about to give it a name, and that fact lives in `store.rs` rather than anywhere in this diff. The `link` window is harder still, because nothing about the name `incoming` suggests index entries pass through it. A review that finds either window has this entry; finding both is a strong review.

Trigger

Not recorded

Severity

blocking