Is it safe to attach a small magnet inside my PC case?

You should be safe with a weak magnet mounted to the case. Avoid placing a powerful neodymium magnet directly against electronics or storage drives, but the distance you described from the power supply and wiring is unlikely to cause a problem.

A small, ordinary decorative or refrigerator-style magnet should be fine, especially if it isn’t touching any components. PC cases already commonly use magnets for fan filters, side panels, and LED accessories. The main thing to be cautious around is a mechanical hard drive, since magnets can potentially affect its data or operation. Keep stronger neodymium magnets away from it.

A normal fridge magnet isn’t something I’d worry about in that location. Just make sure it’s secured so it can’t fall into a fan, block airflow, or come loose and short anything. Strong magnets are a different story, so I’d avoid those inside the case.