Add configuration management with Viper

- Implement pkg/config package with Viper integration
- Support environment variables with DLC_ prefix
- Add configurable server host, port, and shutdown timeout
- Update server to use configuration values
- Add config.example.yaml template file
- Update AGENTS.md with configuration documentation
- Maintain backward compatibility with default values

Configuration options:
- DLC_SERVER_HOST (default: 0.0.0.0)
- DLC_SERVER_PORT (default: 8080)
- DLC_SHUTDOWN_TIMEOUT (default: 30s)

pkg/config/config.go

@@ -0,0 +1,57 @@
+package config
+
+import (
+	"fmt"
+	"time"
+
+	"github.com/rs/zerolog/log"
+	"github.com/spf13/viper"
+)
+
+// Config represents the application configuration
+type Config struct {
+	Server struct {
+		Host string `mapstructure:"host"`
+		Port int    `mapstructure:"port"`
+	}
+	Shutdown struct {
+		Timeout time.Duration `mapstructure:"timeout"`
+	}
+}
+
+// LoadConfig loads configuration from environment variables and defaults
+func LoadConfig() (*Config, error) {
+	v := viper.New()
+	
+	// Set default values
+	v.SetDefault("server.host", "0.0.0.0")
+	v.SetDefault("server.port", 8080)
+	v.SetDefault("shutdown.timeout", 30*time.Second)
+	
+	// Bind environment variables
+	v.AutomaticEnv()
+	v.SetEnvPrefix("DLC") // DanceLessonsCoach prefix
+	v.BindEnv("server.host", "DLC_SERVER_HOST")
+	v.BindEnv("server.port", "DLC_SERVER_PORT")
+	v.BindEnv("shutdown.timeout", "DLC_SHUTDOWN_TIMEOUT")
+	
+	// Unmarshal into Config struct
+	var config Config
+	if err := v.Unmarshal(&config); err != nil {
+		log.Error().Err(err).Msg("Failed to unmarshal config")
+		return nil, fmt.Errorf("config unmarshal error: %w", err)
+	}
+	
+	log.Info().
+		Str("host", config.Server.Host).
+		Int("port", config.Server.Port).
+		Dur("shutdown_timeout", config.Shutdown.Timeout).
+		Msg("Configuration loaded")
+	
+	return &config, nil
+}
+
+// GetServerAddress returns the formatted server address (host:port)
+func (c *Config) GetServerAddress() string {
+	return fmt.Sprintf("%s:%d", c.Server.Host, c.Server.Port)
+}