perf(database): index BatchTaskRun on (runtimeEnvironmentId, createdAt, id) for the batches list (#4361)

## Summary

The batches list page orders by `createdAt DESC, id DESC` filtered by
environment and a created-at window, but the only supporting index on
`BatchTaskRun` was `(runtimeEnvironmentId, id)`. That index can't
satisfy the `createdAt` ordering, so on environments with a large number
of batches the query fell back to a full table scan and in-memory sort,
which could run long enough to hit the statement timeout.

## Fix

Adds `(runtimeEnvironmentId, createdAt DESC, id DESC)` on
`BatchTaskRun`. The query now reads straight from the index in order
with no sort step, returning a page with only a handful of heap fetches
instead of scanning the whole environment slice.

The migration uses `CREATE INDEX CONCURRENTLY IF NOT EXISTS`, so it
takes no table lock and is a no-op if the index already exists.

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 {