Just fitted new brake discs and already seeing light surface rust after a week of rain. Is this normal for fresh ones, or did I get bad quality? How do you prevent it long-term?

Light surface rust is pretty common on new discs, especially uncoated ones- it usually wears off after a few drives as pads scrub it. But if it's heavy or pitting, quality might be off. I prefer coated versions now to minimize that. Found a good bunch of coated and vented options at https://elartcom.eu/categories/brake-discs-2 - they're designed for European weather, and the coating holds up much better. Mine from there stayed almost rust-free through winter. Just clean wheels regularly and avoid long parking in wet conditions. Makes a noticeable difference in longevity.