Add comprehensive godoc documentation to all packages

- Add package-level documentation with usage examples and architecture details
- Document all public types, functions, and methods following godoc conventions
- Remove unused logger.Error type and NewError function
- Apply consistent documentation style across all packages

Packages updated:
- apitls: TLS certificate management with automatic renewal
- config: Environment-based configuration system
- config/depenv: Deployment environment handling
- ekko: Enhanced Echo web framework wrapper
- kafka: Kafka client wrapper with TLS support
- logger: Structured logging with OpenTelemetry integration
- tracing: OpenTelemetry distributed tracing setup
- types: Shared data structures for NTP Pool project
- xff/fastlyxff: Fastly CDN IP range management

All tests pass after documentation changes.

config/depenv/depenv.go

@@ -1,3 +1,19 @@
+// Package depenv provides deployment environment management for NTP Pool services.
+//
+// This package handles different deployment environments (development, test, production)
+// and provides environment-specific configuration including API endpoints, management URLs,
+// and monitoring domains. It supports string-based environment identification and
+// automatic URL construction for various service endpoints.
+//
+// The package defines three main deployment environments:
+//   - DeployDevel: Development environment with dev-specific endpoints
+//   - DeployTest: Test/beta environment for staging
+//   - DeployProd: Production environment with live endpoints
+//
+// Environment detection supports both short and long forms:
+//   - "dev" or "devel" → DeployDevel
+//   - "test" or "beta" → DeployTest
+//   - "prod" → DeployProd
 package depenv
 
 import (
@@ -24,14 +40,27 @@ var apiServers = map[DeploymentEnvironment]string{
 // }
 
 const (
+	// DeployUndefined represents an unrecognized or unset deployment environment.
 	DeployUndefined DeploymentEnvironment = iota
+	// DeployDevel represents the development environment.
 	DeployDevel
+	// DeployTest represents the test/beta environment.
 	DeployTest
+	// DeployProd represents the production environment.
 	DeployProd
 )
 
+// DeploymentEnvironment represents a deployment environment type.
+// It provides methods for environment-specific URL construction and
+// supports text marshaling/unmarshaling for configuration files.
 type DeploymentEnvironment uint8
 
+// DeploymentEnvironmentFromString parses a string into a DeploymentEnvironment.
+// It supports both short and long forms of environment names:
+//   - "dev" or "devel" → DeployDevel
+//   - "test" or "beta" → DeployTest
+//   - "prod" → DeployProd
+//   - any other value → DeployUndefined
 func DeploymentEnvironmentFromString(s string) DeploymentEnvironment {
 	switch s {
 	case "devel", "dev":
@@ -45,6 +74,8 @@ func DeploymentEnvironmentFromString(s string) DeploymentEnvironment {
 	}
 }
 
+// String returns the canonical string representation of the deployment environment.
+// Returns "prod", "test", "devel", or panics for invalid environments.
 func (d DeploymentEnvironment) String() string {
 	switch d {
 	case DeployProd:
@@ -58,6 +89,9 @@ func (d DeploymentEnvironment) String() string {
 	}
 }
 
+// APIHost returns the API server URL for this deployment environment.
+// It first checks the API_HOST environment variable for overrides,
+// then falls back to the environment-specific default API endpoint.
 func (d DeploymentEnvironment) APIHost() string {
 	if apiHost := os.Getenv("API_HOST"); apiHost != "" {
 		return apiHost
@@ -65,14 +99,26 @@ func (d DeploymentEnvironment) APIHost() string {
 	return apiServers[d]
 }
 
+// ManageURL constructs a management interface URL for this deployment environment.
+// It combines the environment-specific management server base URL with the provided path.
+//
+// The path parameter should start with "/" for proper URL construction.
 func (d DeploymentEnvironment) ManageURL(path string) string {
 	return manageServers[d] + path
 }
 
+// MonitorDomain returns the monitoring domain for this deployment environment.
+// The domain follows the pattern: {environment}.mon.ntppool.dev
+// For example: "devel.mon.ntppool.dev" for the development environment.
 func (d DeploymentEnvironment) MonitorDomain() string {
 	return d.String() + ".mon.ntppool.dev"
 }
 
+// UnmarshalText implements the encoding.TextUnmarshaler interface.
+// It allows DeploymentEnvironment to be unmarshaled from configuration files
+// and other text-based formats. Empty strings are treated as valid (no-op).
+//
+// Returns an error if the text represents an invalid deployment environment.
 func (d *DeploymentEnvironment) UnmarshalText(text []byte) error {
 	s := string(text)
 	if s == "" {