feat(registry): add service discovery and health check capabilities

- Integrate tonic-health for gRPC service health monitoring
- Add etcd-based service registration with automatic keep-alive
- Implement dynamic configuration loading from etcd with fallback
- Remove external dependencies from docker-compose for simplified deployment
- Refactor service registration logic with improved lease management
- Add health service to gRPC server with serving status reporting

etcd/mod.rs

@@ -116,6 +116,32 @@ impl EtcdRegistry {
         ids.sort();
         ids
     }
+
+    /// Read config from etcd. Priority: etcd > env > default.
+    /// This is async but can be called from sync context via block_on.
+    pub async fn get_config(&self, key: &str, default: &str) -> String {
+        let etcd_key = format!("{}config/{}", self.inner.key_prefix, key);
+        let mut client = self.inner.client.lock().await;
+        if let Ok(resp) = client.get(etcd_key.as_str(), None).await {
+            if let Some(kv) = resp.kvs().first() {
+                if let Ok(v) = kv.value_str() {
+                    if !v.is_empty() {
+                        tracing::info!(key, value = v, "config from etcd");
+                        return v.to_string();
+                    }
+                }
+            }
+        }
+        drop(client);
+        // Fall back to env
+        if let Ok(v) = std::env::var(key) {
+            if !v.is_empty() {
+                return v;
+            }
+        }
+        default.to_string()
+    }
+
 }
 
 /// Derive a deterministic UUID from a gitks storage_name.