Add /analytics/users/retention endpoint

[phertyameen]

Description

Add a controller route GET /analytics/users/retention exposing the retention curve. Validate query params with a DTO (class-validator), guard the route with AnalyticsAdminGuard where the data is sensitive, and delegate the computation to get-retention-curve.provider.ts.

Why this matters

This is the API surface the analytics dashboard (frontend) and any external reporting tools will consume for 'the retention curve'.

Files to touch

• backend/src/analytics/analytics.controller.ts
• backend/src/analytics/providers/get-retention-curve.provider.ts
• backend/src/analytics/dtos/analytics-query.dto.ts

Acceptance Criteria

• GET /analytics/users/retention returns a 200 with the documented response shape
• Invalid query params return a 400 with a clear validation message
• Route is covered by an e2e test in backend/test/analytics.e2e-spec.ts

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).

[abdoolyaro]

Hi, I’d like to take this issue. I can implement the retention curve endpoint, add DTO-based query validation, protect it with the appropriate admin guard, and include e2e tests to verify the API behavior.

[grantfox-oss]

🦊 GrantFox — @abdoolyaro 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 #527)
2. Your PR will be reviewed by the MindBlockLabs maintainers

Good luck! Track your progress on GrantFox.