[dataviz] perf: load reco with several HTTP requests

[GresilleSiffle]

related issues #1867

---

Description

The page "Videos and channels" now retrieves the recommendations from the API /polls/ instead of the legacy API /video/.

Instead of retrieving up to 99999 entities in one single HTTP request, the new method uses a thread pool and several requests to fetch batches of 2000 entities. The new method takes ~ 28 seconds vs ~ 50 seconds for the previous one.

We can tweak several parameters to adapt the server workload:

• lowering the query parameter limit will reduce the execution time per request but will increase the total number of requests (and vice versa)
• lowering the number of worker threads will reduce the number of concurrent HTTP requests and increase the loading time of the Streamlit page

Let me know if you think 2000 is too low or too high.

Checklist

• I added the related issue(s) id in the related issues section (if any)
  • if not, delete the related issues section
• I described my changes and my decisions in the PR description
• I read the development guidelines of the CONTRIBUTING.md
• The tests pass and have been updated if relevant
• The code quality check pass