Prober: the signature index by the poster's md5

A poster of 2.7 kB broke the index build (a btree key is at most a third of a page) and left an invalid index
that IF NOT EXISTS would have taken for the real one: the migration drops it first and indexes md5(poster), and
the sweep asks by it.

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

database/migrations/1770000007200_pending_signature_index.ts

@@ -3,14 +3,17 @@ 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.
+ * lanes' claims (LOG.md (134)). The waiting rows by signature make it an index range. By the poster's md5: a
+ * poster of 2.7 kB broke the first build (a btree key is at most a third of a page), which left an invalid index
+ * behind that IF NOT EXISTS would have taken for the real one; the drop clears it.
  */
 export default class extends BaseSchema {
   static disableTransactions = true
 
   async up() {
+    this.schema.raw(`DROP INDEX CONCURRENTLY IF EXISTS pending_releases_signature_idx`)
     this.schema.raw(
-      `CREATE INDEX CONCURRENTLY IF NOT EXISTS pending_releases_signature_idx ON pending_releases (group_id, tool, poster) WHERE status = 0`
+      `CREATE INDEX CONCURRENTLY IF NOT EXISTS pending_releases_signature_idx ON pending_releases (group_id, tool, md5(poster)) WHERE status = 0`
     )
   }