Change delayed_jobs_reserve index

Add WHERE condition. Jobs that failed are only kept for debugging
purposes, they are not taken into account when reserving a job. The
corresponding SELECT statement [1] includes 'WHERE failed_at IS NULL',
so the corresponding index can omit failed jobs.

Partial indexes are not supported for MySQL, thus this change is only
for PostgreSQL.

[1] https://github.com/TalentBox/delayed_job_sequel/
      blob/938ff68e98b44bbc391805c672a74c5ee3aef08b/
      lib/delayed/backend/sequel.rb#L26

db/migrations/20240222131500_change_delayed_jobs_reserve_index.rb

@@ -0,0 +1,17 @@
+Sequel.migration do
+  up do
+    if database_type == :postgres
+      drop_index :delayed_jobs, nil, name: :delayed_jobs_reserve, options: %i[if_exists concurrently]
+      add_index :delayed_jobs, %i[queue locked_at locked_by failed_at run_at priority],
+                where: { failed_at: nil }, name: :delayed_jobs_reserve, options: %i[if_not_exists concurrently]
+    end
+  end
+
+  down do
+    if database_type == :postgres
+      drop_index :delayed_jobs, nil, name: :delayed_jobs_reserve, options: %i[if_exists concurrently]
+      add_index :delayed_jobs, %i[queue locked_at locked_by failed_at run_at priority],
+                name: :delayed_jobs_reserve, options: %i[if_not_exists concurrently]
+    end
+  end
+end