Fix account delete blocked by CsvUpload FK constraint

Add onDelete: Cascade to CsvUpload.accountId so deleting an account
cascades to its upload records. Also explicitly delete uploads before
the account in the API route so existing deployed DBs without the
constraint don't error.

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

src/app/api/accounts/[id]/route.ts

@@ -58,6 +58,7 @@ export async function DELETE(_req: Request, { params }: Params) {
   })
   if (!existing) return NextResponse.json({ error: 'Not found' }, { status: 404 })
 
+  await prisma.csvUpload.deleteMany({ where: { accountId: id } })
   await prisma.account.delete({ where: { id } })
   return new NextResponse(null, { status: 204 })
 }