Issue #3 - Added events app: models, serializers, viewsets, and added region field for individual events

[jordansingarajah]

Change Summary

Implements the Django backend for the Events page: 'Event' and 'EventInstance' models,
serializers (with nested instances), viewsets, URL routing, and region/date parameter filtering.

Change Form

Fill this up (NA if not available). If a certain criteria is not met, can you please give a reason.

• [✅] The pull request title has an issue number
• [✅] The change works by "Smoke testing" or quick testing
• The change has tests
  • No automated tests yet, only manual smoke testing for now.
• The change has documentation
  • No separate docs written

Other Information

• 'cover_image' field in the Event model is a CharField storing a relative path (not 'URLField'), since images
  are uploaded directly to the frontend (/public/) rather than hosted externally
• 'region' field added to the EventInstance model, which is a plain CharField with no fixed choices, per client's scalability
  preference - new regions can be added in the future
• branch name renamed to avoid frontend/backend overlap

Related issue

• Resolve Create Events Page #3

[KKatariah]

Flake8 tests are failing.

Warning: server/api/events/admin.py:5:35: W292 no newline at end of file
Error: server/api/events/tests.py:1:1: F401 'django.test.TestCase' imported but unused
Error: The process '/opt/hostedtoolcache/Python/3.12.13/x64/bin/flake8' failed with exit code 1