Applied at property management company for Unknown

I recently went through an interview process with a property management company where they asked me to create a full marketing plan for their worst performing building. This included a detailed rent analysis, competitor research, and suggestions for renovations. I spent around 15 hours over the weekend working on it, really putting in the effort to make it comprehensive. After all that work, they informed me that they decided to go with an 'internal candidate.' A month later, I noticed my exact suggestions posted on their website, which felt incredibly frustrating and like I had been taken advantage of. It's disheartening to see companies ask for so much free labor during the interview process, only to ghost you or use your ideas without any credit.