Photo wall from past meetup #133

As someone considering attending,
I want to see real photos of past meetups,
so that I can gauge the vibe before committing.
AC:

• _data/photos.yml with { meetup_slug, image, caption } entries
• Homepage strip (6–8 curated photos, lazy-loaded, < 200 KB each)
• Alt text required per entry
• Section hidden if photos.yml is empty