The discard sweep beside the lanes, each dropped signature swept once

After aa40222 the prober still idled: a dropped signature's range of the
ladder index is minutes of HDD (309 dropped, 242 with a poster tested on
the rows) and the sweep read the same ranges again every five minutes
inside the claim. Now each dropped class and signature is swept once
(swept_at, migration 7100; null again when the record drops it anew),
20 of each per sweep in rounds of 10k rows, in a loop of its own in the
prober beside the lanes and the namer (LOG.md (133)).

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

database/migrations/1770000007100_pending_swept.ts

@@ -0,0 +1,26 @@
+import { BaseSchema } from '@adonisjs/lucid/schema'
+
+/**
+ * The ladder's discard sweep (PendingLadder.discard) reads a dropped signature's or class's waiting rows once:
+ * `swept_at` marks it done (null again when the record drops it anew). Before, every sweep read every dropped
+ * signature's rows over again, minutes of HDD each, inside the claim (LOG.md (133)).
+ */
+export default class extends BaseSchema {
+  async up() {
+    this.schema.alterTable('pending_signatures', (table) => {
+      table.timestamp('swept_at', { useTz: true }).nullable()
+    })
+    this.schema.alterTable('pending_classes', (table) => {
+      table.timestamp('swept_at', { useTz: true }).nullable()
+    })
+  }
+
+  async down() {
+    this.schema.alterTable('pending_signatures', (table) => {
+      table.dropColumn('swept_at')
+    })
+    this.schema.alterTable('pending_classes', (table) => {
+      table.dropColumn('swept_at')
+    })
+  }
+}