#631 Add tests for achievement unlock conditions

[vic-Gray]

Write tests verifying achievements are unlocked by the correct event triggers.\n\nWhy: Incorrectly awarded achievements break user trust.\n\nHints:\n- `backend/src/achievements/achievements.service.ts

[raphael-01-visual]

@raphael-01-visual has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

Can I work on this issue ? ETA - 12hrs

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[drips-wave]

Congratulations, @raphael-01-visual! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 2, 2026.

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