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/leader.go

@@ -44,8 +44,8 @@ func NewLeaderElection(nodeID string, natsConn *nats.Conn, callbacks LeaderElect
 		Bucket:      "aether-leader-election",
 		Description: "Aether cluster leader election coordination",
 		TTL:         LeaderLeaseTimeout * 2, // Auto-cleanup expired leases
-		MaxBytes:    1024 * 1024,           // 1MB max
-		Replicas:    1,                     // Single replica for simplicity
+		MaxBytes:    1024 * 1024,            // 1MB max
+		Replicas:    1,                      // Single replica for simplicity
 	})
 	if err != nil {
 		// Try to get existing KV store
@@ -411,4 +411,4 @@ func (le *LeaderElection) updateCurrentLeader(leaderID string, term uint64) {
 			le.callbacks.OnNewLeader(leaderID)
 		}
 	}
-}
+}