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>

nats_eventbus.go

@@ -13,11 +13,11 @@ import (
 
 // NATSEventBus is an EventBus that broadcasts events across all cluster nodes using NATS
 type NATSEventBus struct {
-	*EventBus                             // Embed base EventBus for local subscriptions
-	nc                   *nats.Conn       // NATS connection
+	*EventBus                       // Embed base EventBus for local subscriptions
+	nc                   *nats.Conn // NATS connection
 	subscriptions        []*nats.Subscription
-	namespaceSubscribers map[string]int   // Track number of subscribers per namespace
-	nodeID               string           // Unique ID for this node
+	namespaceSubscribers map[string]int // Track number of subscribers per namespace
+	nodeID               string         // Unique ID for this node
 	mutex                sync.Mutex
 	ctx                  context.Context
 	cancel               context.CancelFunc