feat: cache websoc

[KevinWu098]

Summary

1. Reintroduces websoc result caching, which was removed in feat: migrate to Anteater API #1031
2. Updates eslint at root to v9 with the flat config

npx @eslint/migrate-config .eslintrc.cjs

Test Plan

1. Search for a course
2. Switch tabs or otherwise clear the search
3. Searching for the course again should hit the cache (no loading state), and it should also scroll to the appropriate height that you last left off on (feat: mobile ui and saved scroll position #1001 was also regressed by feat: migrate to Anteater API #1031)
4. Searching for a different course should not hit that cache

Issues

Closes #

[MinhxNguyen7]

LGTM!