Implement nightly retention cohort rollup job

[phertyameen]

Description

Add a @Cron job that updates RetentionCohort rows nightly for all cohorts within the tracked window (e.g. last 90 days).

Why this matters

Retention curves need daily recomputation as more users cross day-1/7/30 thresholds relative to their signup cohort.

Files to touch

• backend/src/analytics/jobs/retention-cohort-rollup.job.ts

Acceptance Criteria

• Job updates only affected cohort/day-offset rows, not the full history
• Job completes within an acceptable time budget on seeded test data

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).

[kiel997]

Hi! I'd like to take on this issue. I have experience building backend features with NestJS and TypeScript, including scheduled jobs, analytics, and database operations. I'll implement the nightly retention cohort rollup job with a focus on correctness, performance, and maintainability, ensuring it integrates cleanly with the existing architecture. I'll also add appropriate tests and documentation where needed to ensure the feature is reliable and easy to maintain. I look forward to contributing this implementation.

[grantfox-oss]

🦊 GrantFox — @kiel997 has been assigned to this issue as part of the Official Campaign | FWC26 campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #547)
2. Your PR will be reviewed by the MindBlockLabs maintainers

Good luck! Track your progress on GrantFox.

[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Official Campaign | FWC26 campaign!

@kiel997's PR #607 was approved and merged by @phertyameen.

🏆 @kiel997: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (204 total points). Track your full progress on GrantFox.

👏 Great work, @kiel997! Keep contributing to MindBlockLabs.