Prober: claim range per value of unreadable, from the index ends

Over unreadable >= false Postgres walked every waiting entry of the kind:
2.5 minutes a claim at 28M archive candidates, the prober idle every other
minute. Per value each min and max is one end of the ready index (56 ms).

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

app/services/prober.ts

@@ -167,8 +167,17 @@ export class Prober {
       if (want <= 0) return
       // The starting points chosen here: in the query Postgres drew random() once for all slices.
       // Over the claimable ones of this tier only: the rest of the queue isn't settled yet.
+      // Per value of unreadable, each an end of the index: over `unreadable >= false` Postgres walked every waiting
+      // entry of the kind, 2.5 minutes a claim at 28M archive candidates (LOG.md 2026-10-18 (74)).
+      const values = unreadable ? [true] : [false, true]
+      const ends = values
+        .map(() => `SELECT min(id) AS lo, max(id) AS hi FROM probe_candidates WHERE status = 'waiting' AND ready AND kind = ? AND unreadable = ?`)
+        .join(' UNION ALL ')
       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])
+        await db.rawQuery(
+          `SELECT min(lo) AS lo, max(hi) AS hi FROM (${ends}) e`,
+          values.flatMap((v) => [kind, v])
+        )
       ).rows
       if (lo === null) continue
       const starts = Array.from({ length: SLICES }, () => Number(lo) + Math.floor(Math.random() * (Number(hi) - Number(lo) + 1)))