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/leases.ts

@@ -19,7 +19,7 @@ class Leases {
       `UPDATE ranges r SET status = 'leased', node_id = ?, leased_at = now(), heartbeat_at = now(), attempts = attempts + 1, rule_version = ?
        WHERE r.id = (
          SELECT id FROM ranges
-         WHERE spooled_chunks > 0 AND (status = 'planned' OR (status = 'leased' AND heartbeat_at < now() - make_interval(secs => ?)))
+         WHERE spooled_chunks > 0 AND next_id <= end_id AND (status = 'planned' OR (status = 'leased' AND heartbeat_at < now() - make_interval(secs => ?)))
          ORDER BY priority, id LIMIT 1 FOR UPDATE SKIP LOCKED)
        RETURNING r.id, r.group_id, r.start_id, r.end_id, r.next_id`,
       [nodeId, rules.version, coordinatorConfig.leaseTtlSeconds]

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]) {