🎉🌐 Multiple Events Support 🚀

[TheCommandCat]

This PR adds full support for managing multiple concurrent election events instead of a single global event. Here’s what’s new:

🚀 Features

• 📅 Multi‐Event CRUD
  • Endpoints:
  • GET /api/admin/events
  • POST /api/admin/events
  • PUT /api/admin/events/:eventId
  • DELETE /api/admin/events/:eventId
    • Public:
  • GET /public/events
  • GET /public/events/:eventId
• 🔑 Event‐Scoped Data
  • All collections (members, mm‐members, rounds, votes, cities) now include an eventId field
  • Queries automatically filter by eventId to isolate each event’s data
• 🧑‍💻 UI Changes
  • Admin Events List page for creating, editing, deleting and selecting events
  • Per‐event admin console at /admin/events/:eventId/...
  • Voter UI prompts user to select an active event before joining
• 🔄 Routing & Guards
  • Middleware validates eventId on all API routes
  • Legacy single‐event routes redirect to the new events list

🛠️ Refactors

• 🗃️ Database schema extended: every record now stores eventId: ObjectId
• 🔄 Updated service layers and React hooks to pass and consume eventId
• 📦 Removed hardcoded “single event” logic throughout backend and frontend

✅ Migration & Backwards Compatibility

• Existing data is back‐filled with a default eventId so single‐event setups continue working
• Admin flows fall back to the “default” event if no other events exist

Closes #8