Prober: claim only candidates of settled releases, without the join

The settler marks a settled release's candidates ready (and unreadable when
it had no readable name). Claims took the candidates by joining releases
from random points of the queue; in its newer half nothing had settled, so
a slice scanned to the end (0.3 s each, 20 slices and 5 tiers a claim) and
the prober stood still: 2.4M waiting, 0 probed a minute. Now an index over
the claimable rows only, the slices drawn within them.

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

app/services/prober.ts

@@ -166,15 +166,19 @@ export class Prober {
       const want = coordinatorConfig.probe.batch - this.queue.length
       if (want <= 0) return
       // The starting points chosen here: in the query Postgres drew random() once for all slices.
-      const [{ lo, hi }] = (await db.rawQuery(`SELECT min(id) AS lo, max(id) AS hi FROM probe_candidates`)).rows
+      // Over the claimable ones of this tier only: the rest of the queue isn't settled yet.
+      const [{ lo, hi }] = (
+        await db.rawQuery(`SELECT min(id) AS lo, max(id) AS hi FROM probe_candidates WHERE status = 'waiting' AND ready AND kind = ? AND unreadable >= ?`, [kind, unreadable])
+      ).rows
+      if (lo === null) continue
       const starts = Array.from({ length: SLICES }, () => Number(lo) + Math.floor(Math.random() * (Number(hi) - Number(lo) + 1)))
       const { rows } = await db.rawQuery(
         `UPDATE probe_candidates SET status = 'taken' WHERE id IN (
            SELECT pc.id FROM unnest(?::bigint[]) AS o(start)
            CROSS JOIN LATERAL (
-             SELECT pc.id FROM probe_candidates pc JOIN releases r ON r.id = pc.release_id
-             WHERE pc.status = 'waiting' AND pc.kind = ? AND r.settled_at IS NOT NULL AND (NOT ? OR NOT r.readable) AND pc.id >= o.start
-             ORDER BY pc.id LIMIT ? FOR UPDATE OF pc SKIP LOCKED) pc)
+             SELECT pc.id FROM probe_candidates pc
+             WHERE pc.status = 'waiting' AND pc.ready AND pc.kind = ? AND pc.unreadable >= ? AND pc.id >= o.start
+             ORDER BY pc.kind, pc.unreadable, pc.id LIMIT ? FOR UPDATE SKIP LOCKED) pc)
          RETURNING id, release_id, kind, message_id`,
         [starts, kind, unreadable, Math.ceil(want / SLICES)]
       )