Add mutex protection to ConsistentHashRing for thread safety

- Add sync.RWMutex to ConsistentHashRing struct
- Use Lock/Unlock for write operations (AddNode, RemoveNode)
- Use RLock/RUnlock for read operations (GetNode, GetNodes, IsEmpty)

This allows concurrent reads (the common case) while serializing writes,
preventing race conditions when multiple goroutines access the hash ring.

Closes #35

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

cluster/hashring.go

@@ -5,10 +5,12 @@ import (
 	"encoding/binary"
 	"fmt"
 	"sort"
+	"sync"
 )
 
 // ConsistentHashRing implements a consistent hash ring for shard distribution
 type ConsistentHashRing struct {
+	mu           sync.RWMutex
 	ring         map[uint32]string // hash -> node ID
 	sortedHashes []uint32          // sorted hash keys
 	nodes        map[string]bool   // active nodes
@@ -35,6 +37,9 @@ func NewConsistentHashRingWithConfig(config HashRingConfig) *ConsistentHashRing
 
 // AddNode adds a node to the hash ring
 func (chr *ConsistentHashRing) AddNode(nodeID string) {
+	chr.mu.Lock()
+	defer chr.mu.Unlock()
+
 	if chr.nodes[nodeID] {
 		return // Node already exists
 	}
@@ -56,6 +61,9 @@ func (chr *ConsistentHashRing) AddNode(nodeID string) {
 
 // RemoveNode removes a node from the hash ring
 func (chr *ConsistentHashRing) RemoveNode(nodeID string) {
+	chr.mu.Lock()
+	defer chr.mu.Unlock()
+
 	if !chr.nodes[nodeID] {
 		return // Node doesn't exist
 	}
@@ -76,6 +84,9 @@ func (chr *ConsistentHashRing) RemoveNode(nodeID string) {
 
 // GetNode returns the node responsible for a given key
 func (chr *ConsistentHashRing) GetNode(key string) string {
+	chr.mu.RLock()
+	defer chr.mu.RUnlock()
+
 	if len(chr.sortedHashes) == 0 {
 		return ""
 	}
@@ -103,6 +114,9 @@ func (chr *ConsistentHashRing) hash(key string) uint32 {
 
 // GetNodes returns all active nodes in the ring
 func (chr *ConsistentHashRing) GetNodes() []string {
+	chr.mu.RLock()
+	defer chr.mu.RUnlock()
+
 	nodes := make([]string, 0, len(chr.nodes))
 	for nodeID := range chr.nodes {
 		nodes = append(nodes, nodeID)
@@ -112,6 +126,9 @@ func (chr *ConsistentHashRing) GetNodes() []string {
 
 // IsEmpty returns true if the ring has no nodes
 func (chr *ConsistentHashRing) IsEmpty() bool {
+	chr.mu.RLock()
+	defer chr.mu.RUnlock()
+
 	return len(chr.nodes) == 0
 }