Prober: the sweep by a signature index, pending rows locked in one order, deadlocked flushes kept

The discard sweep of a dropped signature read the whole pending_releases table per signature (a parallel seq
scan over the poster test, four minutes on the HDD that starved the lanes' claims): a partial index on
(group_id, tool, poster) WHERE status = 0 makes it a range, and the sweep queries a poster's rows by it and a
tool's and group's by the ladder index.

The batch store's pending upsert, the record, and the sweep deadlocked on pending_releases and pending_classes
rows locked in different orders, and a deadlocked flush dropped its outcomes: lockOrder() sorts every
multi-row statement's set the same way, and a flush that deadlocks anyway keeps its outcomes for the next.

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

database/migrations/1770000007200_pending_signature_index.ts

@@ -0,0 +1,20 @@
+import { BaseSchema } from '@adonisjs/lucid/schema'
+
+/**
+ * The discard sweep of a dropped signature (tool, group, poster) read the whole pending_releases table per
+ * signature: the planner took a parallel seq scan over the poster test, four minutes on the HDD that starved the
+ * lanes' claims (LOG.md (134)). The waiting rows by signature make it an index range.
+ */
+export default class extends BaseSchema {
+  static disableTransactions = true
+
+  async up() {
+    this.schema.raw(
+      `CREATE INDEX CONCURRENTLY IF NOT EXISTS pending_releases_signature_idx ON pending_releases (group_id, tool, poster) WHERE status = 0`
+    )
+  }
+
+  async down() {
+    this.schema.raw(`DROP INDEX CONCURRENTLY IF EXISTS pending_releases_signature_idx`)
+  }
+}