feat: add get_active_events_count view

[JTKaduma]

Summary

This PR adds a new public contract view, get_active_events_count(env: Env) -> u32, to expose the number of currently active events on the platform. This gives frontend clients a simple way to display platform activity metrics without having to fetch and filter all events client-side.

What Changed

• Added get_active_events_count(env: Env) -> u32 to the event registry contract
• Introduced a persistent GlobalActiveEventCount storage key
• Wired active event count updates into event storage lifecycle logic so the counter stays accurate when events are:
  • registered
  • activated/deactivated
  • cancelled
  • archived
• Added a unit test covering active count accuracy across these state transitions

Why

Frontends need a lightweight way to show platform activity metrics. Returning the active event count directly from the contract avoids extra client work and keeps the query efficient.

Testing

• Added unit test for active event count tracking
• Verified with:

cargo test -p event-registry

Closes #315

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