Retire the `speaker_avatar` portrait fallback in favour of registry `image`

[duxi90]

lib/speakers.ts now has an image field on Speaker — a portrait per person, which is what the speaker block and performer.image actually want.

Alongside it, withEventAvatar falls back to the event's speaker_avatar when an event has exactly one speaker and that speaker has no registry image. That fallback exists only so the three events that already carry a banner portrait show one without any new uploads. It is restricted to single-speaker events because speaker_avatar is keyed by event slug, so on a panel it identifies nobody.

To close this

1. Move the three existing portraits to per-speaker keys: https://images.cppserbia.org/speakers/{key}.png
   • events/2026-05-27-Cpp-for-Combinatorial-Optimization.md → luka-matijevic
   • events/2026-06-25-Least-Frequently-Used-Cache.md → sergei-blinov
   • events/2026-07-22-The-Story-of-Cpp.md → no speaker: (documentary screening); decide whether it needs one at all
2. Add image: to those registry entries.
3. Delete withEventAvatar and its tests, and drop speaker_avatar from EventHeader in lib/events-server.ts.
4. Consider having scripts/upload-speaker-avatar.ts write the registry entry instead of event frontmatter, so /banner-avatar feeds both the banner and the page.

speaker_avatar itself stays — the banner generator uses it as the featured photo for that event.