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/cluster.go

@@ -44,5 +44,4 @@
 // - Leader election ensures coordination continues despite node failures
 // - Actor migration allows rebalancing when cluster topology changes
 // - Graceful shutdown with proper resource cleanup
-//
-package cluster
+package cluster