fix(ratelimiter): resolve data race in token bucket

[its-jojoo]

Description

Fixes a data race in the token bucket rate limiter where multiple goroutines could concurrently write to entry.LastSeen without synchronization.

Problem

The Go race detector flagged concurrent writes in getEntry():

• Multiple goroutines writing to the same memory location
• No synchronization between writes
• Cleanup goroutine reading while others write

Solution

• Changed LastSeen from time.Time to int64 (Unix nanoseconds) for atomic operations
• Used atomic.StoreInt64 for writes and atomic.LoadInt64 for reads
• Improved getEntry() with RLock for better read performance
• Added double-check pattern to prevent duplicate entries

Testing

• ✅ Verified with go test -race - no data races detected
• ✅ Tested with 50+ concurrent goroutines

Performance Impact

Neutral to positive - uses RLock for reads instead of full Lock

[its-jojoo]