feat(config): integrate etcd-based configuration management

- Add etcd-client dependency with TLS support
- Implement EtcdConfig struct for reading config values with priority: etcd > env > default
- Add ServiceRegistry for service discovery registration in etcd
- Create from_etcd method in AppConfig for loading SMTP configuration
- Update main.rs to use etcd-based config loading with fallback mechanism
- Add etcd module with client connection and key-value operations
- Modify Dockerfile to use cargo-chef for faster builds
- Add docker-compose.yaml for emailks service deployment
- Include AGENTS.md with development guidelines and best practices
- Add build.sh script for podman-based container building
- Update dependencies in Cargo.toml and Cargo.lock

Cargo.toml

@@ -12,9 +12,12 @@ name = "emailks"
 path = "main.rs"
 [dependencies]
 dotenvy = "0.15"
+etcd-client = { version = "0.18", features = ["tls"] }
 lettre = { version = "0.11", features = ["tokio1-native-tls"] }
 prost = "0.14"
 prost-types = "0.14"
+serde = { version = "1", features = ["derive"] }
+serde_json = "1"
 tokio = { version = "1", features = ["rt-multi-thread", "macros", "sync", "signal"] }
 tokio-stream = { version = "0.1", features = ["sync"] }
 tonic = "0.14"
@@ -22,6 +25,7 @@ tonic-health = "0.14"
 tonic-prost = "0.14"
 tracing = "0.1"
 tracing-subscriber = { version = "0.3", features = ["env-filter"] }
+uuid = { version = "1", features = ["v7"] }
 
 [build-dependencies]
 tonic-prost-build = "0.14"