feat(config): integrate etcd for service discovery and config management

- Add etcd-client dependency for distributed configuration storage
- Implement EtcdConfig with priority: etcd > environment variables > defaults
- Add ServiceRegistry for service registration with lease keep-alive
- Integrate etcd-based service discovery for appks gRPC connections
- Add service watcher for real-time service instance updates
- Migrate Redis configuration from single URL to cluster node list
- Update Dockerfile with default IMKS_HOST and IMKS_PORT environment variables
- Add etcd bootstrap configuration through environment variables
- Implement Redis cluster URL building with optional authentication

Cargo.lock

@@ -939,6 +939,24 @@ dependencies = [
  "windows-sys 0.61.2",
 ]
 
+[[package]]
+name = "etcd-client"
+version = "0.18.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "5ed900ba953ca6bf1fadb75e0c6b73d8463b9e2bb6bdb7b4573e8e7295852fbe"
+dependencies = [
+ "http 1.4.2",
+ "prost",
+ "tokio",
+ "tokio-stream",
+ "tonic",
+ "tonic-build",
+ "tonic-prost",
+ "tonic-prost-build",
+ "tower",
+ "tower-service",
+]
+
 [[package]]
 name = "etcetera"
 version = "0.11.0"
@@ -1612,6 +1630,7 @@ dependencies = [
  "base64",
  "chrono",
  "dashmap",
+ "etcd-client",
  "fred",
  "futures-util",
  "jsonwebtoken",
@@ -1629,6 +1648,7 @@ dependencies = [
  "sqlx",
  "thiserror 2.0.18",
  "tokio",
+ "tokio-stream",
  "tonic",
  "tonic-build",
  "tonic-health",