feat(ST-1112): wire story-promotion into saveTask and PATCH /api/tasks/:id

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/api/tasks/[id]/route.ts

@@ -2,6 +2,7 @@ import { authenticateApiRequest } from '@/lib/api-auth'
 import { prisma } from '@/lib/prisma'
 import { z } from 'zod'
 import { TASK_STATUS_API_VALUES, taskStatusFromApi, taskStatusToApi } from '@/lib/task-status'
+import { updateTaskStatusWithStoryPromotion } from '@/lib/tasks-status-update'
 
 // `review` is a valid TaskStatus in the DB and the kanban-board UI, but the
 // sprint task list (components/sprint/task-list.tsx) does not yet render it.
@@ -82,14 +83,28 @@ export async function PATCH(
     }
   }
 
-  const updated = await prisma.task.update({
-    where: { id },
-    data: {
-      ...(dbStatus !== undefined && dbStatus !== null && { status: dbStatus }),
-      ...(parsed.data.implementation_plan !== undefined && {
-        implementation_plan: parsed.data.implementation_plan,
-      }),
-    },
+  const updated = await prisma.$transaction(async (tx) => {
+    const planUpdate = parsed.data.implementation_plan !== undefined
+      ? await tx.task.update({
+          where: { id },
+          data: { implementation_plan: parsed.data.implementation_plan },
+          select: { id: true, status: true, implementation_plan: true },
+        })
+      : null
+
+    if (dbStatus !== undefined && dbStatus !== null) {
+      const result = await updateTaskStatusWithStoryPromotion(id, dbStatus, tx)
+      return {
+        id: result.task.id,
+        status: result.task.status,
+        implementation_plan: result.task.implementation_plan,
+      }
+    }
+
+    if (planUpdate) return planUpdate
+
+    // Should not reach here — patchSchema rejects bodies without status or implementation_plan.
+    throw new Error('Geen wijzigingen')
   })
 
   return Response.json({