fix(registration): prevent duplicate event registrations

[mayurigade-hub]

✦ Description

Prevents duplicate event registrations for the same user.

Added backend validation to check whether a user is already registered for an event before creating a new registration. Also added database-level protection to prevent duplicate user-event registrations and updated the frontend to avoid repeated registration attempts.

Closes #354

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⟡ Type of Change

• Bug fix
• New feature
• Documentation update
• Refactor

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✦ Checklist

• My code follows the project style guidelines.
• I have tested my changes locally.
• No new warnings or errors were introduced.

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⟡ Screenshots / Screen Recordings

Not applicable.

Before	After
Same user could register multiple times for the same event.	Duplicate registrations are blocked.
Participant counts could become inaccurate.	Each user can only have one registration per event.
Multiple API requests could create duplicate records.	Backend validation and database constraints prevent duplicates.

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[github-actions]

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[mayurigade-hub]

Hello @anubhavxdev
Please review this
Thank you!