Rebuild: prefetching no longer waits for merging, nor stalls on finished ranges

The coordinator ran the prefetch tick in the same loop as the merger
(up to 30 s a minute): its connections went idle once their targets
were fetched, and the first leases after a start found nothing spooled.
The prefetcher now ticks in a loop of its own.

The prefetcher took the first planned ranges with nothing spooled, even
ones with nothing left to fetch (next_id past the end): two such ranges
(edge:bench's, set back to planned) kept it from fetching anything. It
and the lease query now skip those, and edge:bench marks its range done.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

app/services/prefetcher.ts

@@ -114,7 +114,7 @@ export class Prefetcher {
     const [{ ready }] = await db.from('ranges').where('status', 'planned').where('spooled_chunks', '>', 0).count('* as ready')
     const room = this.spoolBytes < coordinatorConfig.spool.maxBytes
     const fresh = room && Number(ready) < ahead
-      ? await rows(db.query()).where('r.status', 'planned').where('r.spooled_chunks', 0).orderBy([{ column: 'r.priority' }, { column: 'r.id' }]).limit(ahead - Number(ready))
+      ? await rows(db.query()).where('r.status', 'planned').where('r.spooled_chunks', 0).whereRaw('r.next_id <= r.end_id').orderBy([{ column: 'r.priority' }, { column: 'r.id' }]).limit(ahead - Number(ready))
       : []
     const targets: Target[] = []
     for (const row of [...leased, ...(room ? started : []), ...fresh]) {