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

config.rs

@@ -54,6 +54,43 @@ pub enum SmtpTls {
 }
 
 impl AppConfig {
+    pub fn from_etcd(
+        host: String, port: u16, username: String, password: String,
+        from_email: String, from_name: String, reply_to: String,
+        tls: String, timeout_secs: u64, helo_name: String, allow_request_from: bool,
+        queue_capacity: Option<usize>,
+        listen_addr_str: &str,
+    ) -> Result<Self, ConfigError> {
+        let tls = match tls.trim().to_ascii_lowercase().as_str() {
+            "none" | "false" | "0" => SmtpTls::None,
+            "starttls" | "start_tls" | "start-tls" => SmtpTls::StartTls,
+            "tls" | "ssl" | "smtps" => SmtpTls::Tls,
+            _ => SmtpTls::StartTls,
+        };
+
+        validate_port("PORT", port)?;
+
+        Ok(Self {
+            smtp: SmtpConfig {
+                host,
+                port,
+                username: if username.is_empty() { None } else { Some(username) },
+                password: if password.is_empty() { None } else { Some(password) },
+                from_email: if from_email.is_empty() { None } else { Some(from_email) },
+                from_name: if from_name.is_empty() { None } else { Some(from_name) },
+                reply_to: if reply_to.is_empty() { None } else { Some(reply_to) },
+                tls,
+                timeout: std::time::Duration::from_secs(timeout_secs),
+                helo_name: if helo_name.is_empty() { None } else { Some(helo_name) },
+                allow_request_from,
+            },
+            queue_capacity,
+            listen_addr: listen_addr_str
+                .parse::<std::net::SocketAddr>()
+                .map_err(|e: std::net::AddrParseError| ConfigError::InvalidEnv { name: "LISTEN_ADDR", reason: e.to_string() })?,
+        })
+    }
+
     pub fn from_env() -> Result<Self, ConfigError> {
         let _ = dotenvy::dotenv();