feat(client): add caching for GetSeats API to reduce rate limit errors

[hystking]

Summary

• Add in-memory cache for /seats/ API responses within the Client struct
• Cache is thread-safe using sync.RWMutex with double-checked locking pattern
• Cache is automatically invalidated after AssignSeat() or UnassignSeat() operations
• Reduces API calls during terraform apply to avoid rate limit errors

Test plan

• Run make build to verify compilation
• Run terraform apply with multiple coderabbit_seats resources and confirm /seats/ API is called only once
• Verify seat assignment/unassignment still works correctly (cache invalidation)