Add audit logging for analytics export actions

[phertyameen]

Description

Persist a row to AnalyticsExportLog every time GET /analytics/export is called, capturing who exported what data and for which date range.

Why this matters

Data exports are the highest-risk analytics feature from a privacy standpoint and should be auditable after the fact.

Files to touch

• backend/src/analytics/providers/export-csv.provider.ts
• backend/src/analytics/entities/analytics-export-log.entity.ts

Acceptance Criteria

• Every successful export creates exactly one audit row
• Failed export attempts are also logged with a failure reason

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).