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/types.go

@@ -4,17 +4,47 @@ import (
 	"time"
 )
 
+// Default configuration values
 const (
-	// NumShards defines the total number of shards in the cluster
-	NumShards = 1024
-	// VirtualNodes defines the number of virtual nodes per physical node for consistent hashing
-	VirtualNodes = 150
+	// DefaultNumShards defines the default total number of shards in the cluster
+	DefaultNumShards = 1024
+	// DefaultVirtualNodes defines the default number of virtual nodes per physical node
+	DefaultVirtualNodes = 150
 	// Leadership election constants
 	LeaderLeaseTimeout = 10 * time.Second // How long a leader lease lasts
 	HeartbeatInterval  = 3 * time.Second  // How often leader sends heartbeats
 	ElectionTimeout    = 2 * time.Second  // How long to wait for election
 )
 
+// HashRingConfig holds configuration for the consistent hash ring
+type HashRingConfig struct {
+	// VirtualNodes is the number of virtual nodes per physical node (default: 150)
+	VirtualNodes int
+}
+
+// DefaultHashRingConfig returns the default hash ring configuration
+func DefaultHashRingConfig() HashRingConfig {
+	return HashRingConfig{
+		VirtualNodes: DefaultVirtualNodes,
+	}
+}
+
+// ShardConfig holds configuration for shard management
+type ShardConfig struct {
+	// ShardCount is the total number of shards (default: 1024)
+	ShardCount int
+	// ReplicationFactor is the number of replicas per shard (default: 1)
+	ReplicationFactor int
+}
+
+// DefaultShardConfig returns the default shard configuration
+func DefaultShardConfig() ShardConfig {
+	return ShardConfig{
+		ShardCount:        DefaultNumShards,
+		ReplicationFactor: 1,
+	}
+}
+
 // NodeStatus represents the health status of a node
 type NodeStatus string
 
@@ -30,14 +60,14 @@ type NodeInfo struct {
 	Address   string            `json:"address"`
 	Port      int               `json:"port"`
 	Status    NodeStatus        `json:"status"`
-	Capacity  float64           `json:"capacity"`  // Maximum load capacity
-	Load      float64           `json:"load"`      // Current CPU/memory load
-	LastSeen  time.Time         `json:"lastSeen"`  // Last heartbeat timestamp
+	Capacity  float64           `json:"capacity"` // Maximum load capacity
+	Load      float64           `json:"load"`     // Current CPU/memory load
+	LastSeen  time.Time         `json:"lastSeen"` // Last heartbeat timestamp
 	Timestamp time.Time         `json:"timestamp"`
 	Metadata  map[string]string `json:"metadata"`
 	IsLeader  bool              `json:"isLeader"`
-	VMCount   int               `json:"vmCount"`   // Number of VMs on this node
-	ShardIDs  []int             `json:"shardIds"`  // Shards assigned to this node
+	VMCount   int               `json:"vmCount"`  // Number of VMs on this node
+	ShardIDs  []int             `json:"shardIds"` // Shards assigned to this node
 }
 
 // NodeUpdateType represents the type of node update
@@ -57,9 +87,9 @@ type NodeUpdate struct {
 
 // ShardMap represents the distribution of shards across cluster nodes
 type ShardMap struct {
-	Version    uint64              `json:"version"`    // Incremented on each change
-	Shards     map[int][]string    `json:"shards"`     // shard ID -> [primary, replica1, replica2]
-	Nodes      map[string]NodeInfo `json:"nodes"`      // node ID -> node info
+	Version    uint64              `json:"version"` // Incremented on each change
+	Shards     map[int][]string    `json:"shards"`  // shard ID -> [primary, replica1, replica2]
+	Nodes      map[string]NodeInfo `json:"nodes"`   // node ID -> node info
 	UpdateTime time.Time           `json:"updateTime"`
 }
 
@@ -74,23 +104,23 @@ type ClusterMessage struct {
 
 // RebalanceRequest represents a request to rebalance shards
 type RebalanceRequest struct {
-	RequestID   string            `json:"requestId"`
-	FromNode    string            `json:"fromNode"`
-	ToNode      string            `json:"toNode"`
-	ShardIDs    []int             `json:"shardIds"`
-	Reason      string            `json:"reason"`
-	Migrations  []ActorMigration  `json:"migrations"`
+	RequestID  string           `json:"requestId"`
+	FromNode   string           `json:"fromNode"`
+	ToNode     string           `json:"toNode"`
+	ShardIDs   []int            `json:"shardIds"`
+	Reason     string           `json:"reason"`
+	Migrations []ActorMigration `json:"migrations"`
 }
 
 // ActorMigration represents the migration of an actor between nodes
 type ActorMigration struct {
-	ActorID     string            `json:"actorId"`
-	FromNode    string            `json:"fromNode"`
-	ToNode      string            `json:"toNode"`
-	ShardID     int               `json:"shardId"`
-	State       map[string]interface{} `json:"state"`
-	Version     int64             `json:"version"`
-	Status      string            `json:"status"` // "pending", "in_progress", "completed", "failed"
+	ActorID  string                 `json:"actorId"`
+	FromNode string                 `json:"fromNode"`
+	ToNode   string                 `json:"toNode"`
+	ShardID  int                    `json:"shardId"`
+	State    map[string]interface{} `json:"state"`
+	Version  int64                  `json:"version"`
+	Status   string                 `json:"status"` // "pending", "in_progress", "completed", "failed"
 }
 
 // LeaderElectionCallbacks defines callbacks for leadership changes
@@ -107,4 +137,3 @@ type LeadershipLease struct {
 	ExpiresAt time.Time `json:"expiresAt"`
 	StartedAt time.Time `json:"startedAt"`
 }
-