Precache all thumbnails and pinned originals from Library.open (closes #48)

Two background fills start inside open() with no caller input, through the
shared pools (#45) at background priority, so on-demand work always preempts
them. Thumbnails: every file newest first until all are on disk, sharing the
thumbnail pool with thumbnails.ensure. Originals: the pinned set — the
favorites album, then the latest-week window ending at the newest file —
through the content pool. The pinned set is the eviction predicate (#47), so a
pinned original is never evicted and a file that leaves the set (favorite
removed or window moved on a later refresh) becomes an ordinary, evictable
original. A cached file costs one map lookup and no fetch; each refresh re-kicks
the fills to pick up new files and retry failures. Progress via open()
onProgress and status() (thumbnailsCached/Total, originalsCached/Pinned).

Model: opus-4-8

test/cli/backup.test.ts

@@ -142,6 +142,10 @@ const openLibrary = (source: ContentSource): Promise<Library> =>
         cacheDirectory: join(root, "cache"),
         contentSource: source,
         refreshIntervalSeconds: 3600,
+        // These tests count exact fetches; the background precache (#48) would
+        // add its own, so it is off here (it is covered in precache.test.ts).
+        precacheThumbnails: false,
+        precacheOriginals: false,
     });
 
 const readLedger = (