fix(duplicate-day): extract userId before closure to satisfy TS strict null check

session.user is narrowed inside the early-return guard but TypeScript
does not carry that narrowing into the Prisma transaction callback.
Extract to a const immediately after the guard.

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

src/app/api/events/duplicate-day/route.ts

@@ -5,6 +5,7 @@ import { prisma } from "@/lib/prisma";
 export async function POST(req: Request) {
   const session = await auth();
   if (!session?.user?.id) return NextResponse.json({ error: "Non autorisé" }, { status: 401 });
+  const userId = session.user.id;
 
   const { babyId, sourceDate, targetDate } = await req.json();
 
@@ -49,7 +50,7 @@ export async function POST(req: Request) {
       prisma.event.create({
         data: {
           babyId,
-          userId: session.user.id,
+          userId,
           type: ev.type,
           startedAt: new Date(ev.startedAt.getTime() + offsetMs),
           endedAt: ev.endedAt ? new Date(ev.endedAt.getTime() + offsetMs) : null,