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/shard.go

@@ -33,8 +33,26 @@ type ShardManager struct {
 	replication int
 }
 
-// NewShardManager creates a new shard manager
+// NewShardManager creates a new shard manager with default configuration
 func NewShardManager(shardCount, replication int) *ShardManager {
+	return NewShardManagerWithConfig(ShardConfig{
+		ShardCount:        shardCount,
+		ReplicationFactor: replication,
+	})
+}
+
+// NewShardManagerWithConfig creates a new shard manager with custom configuration
+func NewShardManagerWithConfig(config ShardConfig) *ShardManager {
+	// Apply defaults for zero values
+	shardCount := config.ShardCount
+	if shardCount == 0 {
+		shardCount = DefaultNumShards
+	}
+	replication := config.ReplicationFactor
+	if replication == 0 {
+		replication = 1
+	}
+
 	return &ShardManager{
 		shardCount:  shardCount,
 		shardMap:    &ShardMap{Shards: make(map[int][]string), Nodes: make(map[string]NodeInfo)},
@@ -149,6 +167,15 @@ func (sm *ShardManager) GetActorsInShard(shardID int, nodeID string, vmRegistry
 	return actors
 }
 
+// GetShardCount returns the total number of shards
+func (sm *ShardManager) GetShardCount() int {
+	return sm.shardCount
+}
+
+// GetReplicationFactor returns the replication factor
+func (sm *ShardManager) GetReplicationFactor() int {
+	return sm.replication
+}
 
 // ConsistentHashPlacement implements PlacementStrategy using consistent hashing
 type ConsistentHashPlacement struct{}
@@ -185,4 +212,4 @@ func (chp *ConsistentHashPlacement) RebalanceShards(currentMap *ShardMap, nodes
 	// This is a simplified implementation
 	// In practice, this would implement sophisticated rebalancing logic
 	return currentMap, nil
-}
+}