issue 39 create Registration models

[Viloetsisi]

Change Summary

This PR adds the new Registration model to the event_registration app:

• UUID primary key
• ForeignKey to User (1-to-many)
• status enum with choices draft|submitted|paid
• Process fields (registration_category, age_group, tshirt_size, etc.)
• Progress & timestamp fields (current_step, last_saved_at, submitted_at, created_at, updated_at)

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Change Form

• The pull request title includes the issue number (issue-39-Create_model_for_registration)
• The change has been smoke-tested in the Django shell (created User → Registration, verified forward/reverse relations)
• The change has tests
  Reason: No unit tests added in this PR; will cover testing in a follow-up.
• The change has documentation
  Reason: Model fields are self-explanatory; full docs to accompany API/serializer work.

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Other Information

• After merging, run:

  python manage.py makemigrations event_registration
  python manage.py migrate

[MahitGtg]

lgtm