perf(database): index BatchTaskRun on (runtimeEnvironmentId, createdAt, id) for the batches list

The batches list page orders by createdAt DESC, id DESC filtered by
environment and a created-at window, but the only matching index was
(runtimeEnvironmentId, id), which cannot satisfy the createdAt ordering.
On environments with many batches the query fell back to a full table
scan and in-memory sort, which could run long enough to hit the
statement timeout.

Adds (runtimeEnvironmentId, createdAt DESC, id DESC) so the query reads
straight from the index in order. Created with CONCURRENTLY IF NOT
EXISTS, so it takes no table lock and is a no-op if already present.

Author: ericallam

.server-changes/batch-list-created-at-index.md

@@ -0,0 +1,6 @@
+---
+area: webapp
+type: fix
+---
+
+Speed up the Batches list page for environments with a large number of batches, which could previously time out while loading.

internal-packages/database/prisma/migrations/20260724120000_add_batch_task_run_created_at_dashboard_index/migration.sql

@@ -0,0 +1,2 @@
+-- CreateIndex
+CREATE INDEX CONCURRENTLY IF NOT EXISTS "BatchTaskRun_runtimeEnvironmentId_createdAt_id_idx" ON "public"."BatchTaskRun"("runtimeEnvironmentId", "createdAt" DESC, "id" DESC);

internal-packages/database/prisma/schema.prisma

@@ -1935,6 +1935,7 @@ model BatchTaskRun {
   @@index([dependentTaskAttemptId])
   // This is for the batch list dashboard page
   @@index([runtimeEnvironmentId, id(sort: Desc)])
+  @@index([runtimeEnvironmentId, createdAt(sort: Desc), id(sort: Desc)])
 }
 
 enum BatchTaskRunStatus {