feat: organizers, meetups UI, Plausible analytics, and migration tooling

- Add organizer model/API, admin and public organizer pages, meetup cards
- Refresh events/home/contact; add calendar dialog and carousel components
- Optional Plausible via NEXT_PUBLIC_PLAUSIBLE_* env vars in root layout
- Prisma migration, seed updates, baseline-and-migrate script

Made-with: Cursor

backend/src/constants/organizer.ts

@@ -0,0 +1,2 @@
+/** Slug for the default organizer row (seed + migration). */
+export const DEFAULT_ORGANIZER_SLUG = 'belgian-bitcoin-embassy';