Add unit tests for event tracking provider

[phertyameen]

Description

Write unit tests for backend/src/analytics/providers/track-event.provider.ts covering the happy path, an empty-data edge case, and a boundary date-range case, using mocked repositories.

Why this matters

The event tracking provider currently lacks dedicated test coverage, so a regression could ship undetected.

Files to touch

• backend/src/analytics/providers/track-event.provider.spec.ts

Acceptance Criteria

• At least 3 test cases: happy path, empty data, boundary dates
• Repositories are mocked (no real DB connection in unit tests)
• Tests run under npm test with no manual setup

Notes

This issue is part of the new analytics module effort (tracking, aggregation,
and reporting on player/game activity). Please keep new code inside
backend/src/analytics/ following the same module/controller/service/provider
layout used by existing modules (e.g. progress, streak, quests).