fix: normalize XP event timestamps to UTC

[Yungmon22]

Overview

This PR normalizes accepted XP event timestamps to UTC in the gamification XP service, so event times are stored consistently regardless of the offset supplied by the caller. It also adds tests covering offset inputs and default (missing) timestamp inputs.

Related Issue

Changes

🕒 XP Event Timestamp Normalization

• [MODIFY] BackendAcademy/src/gamification/xp.service.ts

  • Normalize accepted event times to UTC before they are recorded, so offset-bearing inputs are converted to a consistent UTC instant.
  • Apply a default timestamp when no event time is supplied.

• [MODIFY] BackendAcademy/src/gamification/xp.types.ts

  • Update the XP event types to reflect the normalized UTC timestamp handling.

• [MODIFY] BackendAcademy/src/gamification/xp.service.spec.ts

  • Add tests for offset timestamp inputs and default timestamp inputs.

Verification Results

npm test -- BackendAcademy/src/gamification/xp.service.spec.ts
✅ XP service spec passed (offset + default timestamp cases)

Acceptance Criteria	Status
Normalize accepted event times to UTC	✅ Offset inputs converted to UTC in xp.service.ts
Test offset timestamp inputs	✅ Covered in xp.service.spec.ts
Test default timestamp inputs	✅ Covered in xp.service.spec.ts

Closes #1001

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