Prober: claim from random points of the queue, not in id order

In id order the prober went group by group, and some groups' articles
take 0.6-3 s each (a.b.zogg 2016; repeated fetches the same, pipelining
4 deep no faster), where a sample across all years takes 20-60 ms for
most: 7 articles/s on 6 connections, against 67/s in its first minutes.
A claim now takes 20 slices from random points (353 ms per 200).
tools/body_timing.ts: BODY time for the oldest and newest candidates.

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

app/services/prober.ts

@@ -19,6 +19,8 @@ type Outcome = { candidate: Candidate; status: 'done' | 'missing' | 'error' | 'w
 /** Kinds in the order they're probed: a random name's real name first, then par2 sets, then archives. */
 const KINDS = ['lone', 'par2', 'archive']
 const FLUSH_MS = 2000
+/** Slices a claim takes from random points of the queue. */
+const SLICES = 20
 const REPORT_MS = 60_000
 
 /**
@@ -114,18 +116,27 @@ export class Prober {
     return this.queue.shift()
   }
 
-  /** Waiting candidates of settled releases, by kind, claimed so another round doesn't take them. */
+  /**
+   * Waiting candidates of settled releases, by kind, claimed so another round
+   * doesn't take them: SLICES slices from random points of the queue. In id
+   * order it went group by group, and some groups' articles take 0.6–3 s
+   * where most take 20–60 ms (a.b.zogg 2016: 7 articles/s on 6 connections).
+   */
   private async claim() {
     for (const kind of KINDS) {
       const want = coordinatorConfig.probe.batch - this.queue.length
       if (want <= 0) return
       const { rows } = await db.rawQuery(
         `UPDATE probe_candidates SET status = 'taken' WHERE id IN (
-           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
-           ORDER BY pc.id LIMIT ? FOR UPDATE OF pc SKIP LOCKED)
+           SELECT pc.id FROM (SELECT min(id) AS lo, max(id) AS hi FROM probe_candidates) b
+           CROSS JOIN LATERAL generate_series(1, ?) AS s
+           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 pc.id >= b.lo + floor(random() * (b.hi - b.lo + 1))::bigint + s * 0
+             ORDER BY pc.id LIMIT ? FOR UPDATE OF pc SKIP LOCKED) pc)
          RETURNING id, release_id, kind, message_id`,
-        [kind, want]
+        [SLICES, kind, Math.ceil(want / SLICES)]
       )
       for (const r of rows) this.queue.push({ id: Number(r.id), releaseId: Number(r.release_id), kind: r.kind, messageId: r.message_id })
     }