fix: bound and evict stale buckets in in-memory rate limiter (Closes #299)

[TheWeirdDee]

Summary of Changes

Fixes unbounded memory growth in \lib/rate-limit.ts\ by introducing bounded bucket eviction (idle pruning + LRU capacity cap) to \InMemoryRateLimitStore.

• Idle Bucket Pruning: Buckets whose tokens have fully refilled and have been inactive for at least \idleTimeoutMs\ (defaults to 60s) are automatically pruned on new bucket creation or capacity checks.
• LRU Capacity Cap: Enforces a \maxBuckets\ capacity cap (defaults to 10,000) using Least-Recently-Used (LRU) eviction when active keys reach capacity under high volume or header spoofing attacks.
• LRU Order Maintenance: Active bucket lookups update \lastAccess\ timestamps and refresh insertion order in JS \Map\ to guarantee true LRU eviction.
• Test Introspection & Isolation: Exported
  esetRateLimitStateForTest()\ and \getRateLimitBucketCountForTest(store)\ helpers.
• Expanded Unit Tests: Added unit tests in \ est/rate-limit.test.ts\ verifying idle eviction, LRU cap eviction, returning evicted keys, and store introspection.
• Documentation: Updated \docs/deployment.md\ single-instance caveat to document the eviction policy.

Closes #299