Make configuration values injectable rather than hardcoded

Add config structs with sensible defaults for tunable parameters:
- JetStreamConfig for stream retention (1 year) and replica count (1)
- HashRingConfig for virtual nodes per physical node (150)
- ShardConfig for shard count (1024) and replication factor (1)

Each component gets a new WithConfig constructor that accepts custom
configuration, while the original constructors continue to work with
defaults. Zero values in configs fall back to defaults for backward
compatibility.

Closes #38

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

cluster/hashring.go

@@ -12,13 +12,24 @@ type ConsistentHashRing struct {
 	ring         map[uint32]string // hash -> node ID
 	sortedHashes []uint32          // sorted hash keys
 	nodes        map[string]bool   // active nodes
+	virtualNodes int               // number of virtual nodes per physical node
 }
 
-// NewConsistentHashRing creates a new consistent hash ring
+// NewConsistentHashRing creates a new consistent hash ring with default configuration
 func NewConsistentHashRing() *ConsistentHashRing {
+	return NewConsistentHashRingWithConfig(DefaultHashRingConfig())
+}
+
+// NewConsistentHashRingWithConfig creates a new consistent hash ring with custom configuration
+func NewConsistentHashRingWithConfig(config HashRingConfig) *ConsistentHashRing {
+	virtualNodes := config.VirtualNodes
+	if virtualNodes == 0 {
+		virtualNodes = DefaultVirtualNodes
+	}
 	return &ConsistentHashRing{
-		ring:  make(map[uint32]string),
-		nodes: make(map[string]bool),
+		ring:         make(map[uint32]string),
+		nodes:        make(map[string]bool),
+		virtualNodes: virtualNodes,
 	}
 }
 
@@ -31,7 +42,7 @@ func (chr *ConsistentHashRing) AddNode(nodeID string) {
 	chr.nodes[nodeID] = true
 
 	// Add virtual nodes for better distribution
-	for i := 0; i < VirtualNodes; i++ {
+	for i := 0; i < chr.virtualNodes; i++ {
 		virtualKey := fmt.Sprintf("%s:%d", nodeID, i)
 		hash := chr.hash(virtualKey)
 		chr.ring[hash] = nodeID
@@ -102,4 +113,9 @@ func (chr *ConsistentHashRing) GetNodes() []string {
 // IsEmpty returns true if the ring has no nodes
 func (chr *ConsistentHashRing) IsEmpty() bool {
 	return len(chr.nodes) == 0
-}
+}
+
+// GetVirtualNodes returns the number of virtual nodes per physical node
+func (chr *ConsistentHashRing) GetVirtualNodes() int {
+	return chr.virtualNodes
+}