backend: implement `/api/analytics/export` endpoint for CSV download

[ShantKhatri]

Summary

The analytics route at apps/backend/src/routes/analytics.ts currently returns JSON view/click data. Extend it with a CSV export endpoint so card owners can download their analytics data.

Context

Product doc section 5.5 specifies an 'Analytics Dashboard' feature. An export endpoint is a natural next step for power users and aligns with DevCard's privacy-first philosophy (data portability).

Tasks

• add a new route GET /api/analytics/export in analytics.ts — authenticated, owned data only.
• query platform link clicks and card views for the authenticated user grouped by date.
• serialize response as CSV using a lightweight library (e.g. fast-csv or manual string builder).
• set Content-Type: text/csv and Content-Disposition: attachment; filename=devcard-analytics.csv headers.
• validate that the requesting user can only export their own data (no IDOR).
• add tests for the endpoint asserting CSV structure and auth guard.

Acceptance Criteria

• authenticated users can download a CSV with columns: date, platform, event_type, count.
• unauthenticated requests return 401.
• another user's export URL returns 403.

Difficulty

medium — involves HTTP file streaming, CSV serialization, and authorization boundary checks.

[SdSarthak]

Hi @nowbind,

I'd like to work on this issue under GirlScript Summer of Code 2026.

Analytics data export is something I've implemented in data engineering contexts before. The approach here is clean — query the authenticated user's click and view data grouped by date, serialize to CSV with fast-csv or a manual builder, and stream the response with correct Content-Disposition and Content-Type headers.

I'll also make sure the IDOR check is solid: only the requesting user's data is ever included, and I'll add integration tests covering both the happy path and the auth boundary cases as described.

Could you please assign this to me?

Thanks!

[PiyushTheProgrammer]

Hey @ShantKhatri, I would like to work on this issue for GSSoC '26. I have strong experience with backend data processing and database queries. I can efficiently implement the CSV export endpoint. Could you please assign this to me?

[ShantKhatri]

Warm Welcome @PiyushTheProgrammer . Assigned.

[PiyushTheProgrammer]

Thanks for assigning this to me, @ShantKhatri! I have implemented the CSV export endpoint with the required tests and raised PR #125. Looking forward to your review; please let me know if any changes are required!