perf: prune rate limiter clients in background

[kunal-9090]

Summary

• move expired rate-limit client pruning out of the request hot path
• start a background ticker per limiter to prune stale entries every few minutes
• keep allow() to O(1) lookup/update work unless max-client eviction is needed

Fixes #55

Validation

• Not run locally: Go toolchain is not installed in this environment.

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Configuration used: defaults

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Run ID: ee565b8e-aa02-4645-ac7f-2dbe95d22e8e

📥 Commits

Reviewing files that changed from the base of the PR and between f0635c5 and 7134895.

📒 Files selected for processing (1)

• beamsync/server.go

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[PranavAgarkar07]

@kunal-9090 This PR has merge conflicts with main that need to be resolved before it can be merged. Could you please merge main into your branch and fix the conflicts?

git fetch origin main
git checkout codex/beamsync-55-background-prune
git merge origin/main
# resolve conflicts in beamsync/server.go
git push