Probe candidates: an index on (kind, id) of the waiting ones for the prober's claims

A slice of a claim filtered ~700k rows by kind through the id index (0.37 s
a slice, 20 slices and up to 5 tiers a claim, the lanes waiting meanwhile).

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

database/migrations/1770000002400_probe_candidates_kind_index.ts

@@ -0,0 +1,19 @@
+import { BaseSchema } from '@adonisjs/lucid/schema'
+
+/**
+ * The prober claims waiting candidates by kind from random points of the
+ * queue (Prober.claim): by the id index alone a slice filtered ~700k rows for
+ * the rarer kinds (0.37 s a slice, 20 slices and up to 5 tiers a claim, the
+ * lanes waiting meanwhile; LOG.md 2026-10-12 (17)). Built concurrently.
+ */
+export default class extends BaseSchema {
+  static disableTransactions = true
+
+  async up() {
+    this.schema.raw(`CREATE INDEX CONCURRENTLY IF NOT EXISTS probe_candidates_kind_waiting_idx ON probe_candidates (kind, id) WHERE status = 'waiting'`)
+  }
+
+  async down() {
+    this.schema.raw(`DROP INDEX CONCURRENTLY IF EXISTS probe_candidates_kind_waiting_idx`)
+  }
+}