Rate Limit: Fix Memory Leak

[keenanjohnson]

Closes #25

[Copilot]

Pull request overview

This PR addresses issue #25 by adding time-based eviction of per-API-key rate limiter entries so the in-memory key map doesn’t grow without bound over time.

Changes:

• Extend ratelimit.Limiter to track lastSeen per key and lazily evict idle keys after a configured TTL.
• Update server wiring to configure the limiter with a 10-minute idle eviction TTL.
• Add tests for idle eviction behavior using an injected clock (no sleeps).

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
main.go	Updates rate limiter construction to include an idle eviction TTL and adjusts the explanatory comment.
internal/ratelimit/ratelimit.go	Implements TTL-based lazy eviction with per-key lastSeen tracking and a sweep-on-access approach.
internal/ratelimit/ratelimit_test.go	Updates existing tests to use a long TTL helper and adds new tests covering eviction/retention semantics.

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