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)

cmd/server/main.go

@@ -7,13 +7,19 @@ import (
 	"os"
 	"os/signal"
 	"syscall"
-	"time"
 
+	"DanceLessonsCoach/pkg/config"
 	"DanceLessonsCoach/pkg/server"
 	"github.com/rs/zerolog/log"
 )
 
 func main() {
+	// Load configuration
+	cfg, err := config.LoadConfig()
+	if err != nil {
+		log.Fatal().Err(err).Msg("Failed to load configuration")
+	}
+
 	// Create root context with cancellation
 	ctx, cancel := context.WithCancel(context.Background())
 	defer cancel()
@@ -27,11 +33,11 @@ func main() {
 	serverCtx, serverStop := context.WithCancel(ctx)
 	
 	go func() {
-		fmt.Println("Server running on :8080")
-		log.Info().Msg("Starting HTTP server on :8080")
+		fmt.Printf("Server running on %s\n", cfg.GetServerAddress())
+		log.Info().Str("address", cfg.GetServerAddress()).Msg("Starting HTTP server")
 		
 		srv := &http.Server{
-			Addr:    ":8080",
+			Addr:    cfg.GetServerAddress(),
 			Handler: server.Router(),
 		}
 		
@@ -40,8 +46,8 @@ func main() {
 			<-sigChan
 			log.Info().Msg("Shutdown signal received")
 			
-			// Create shutdown context with timeout
-			shutdownCtx, shutdownCancel := context.WithTimeout(context.Background(), 30*time.Second)
+			// Create shutdown context with timeout from config
+			shutdownCtx, shutdownCancel := context.WithTimeout(context.Background(), cfg.Shutdown.Timeout)
 			defer shutdownCancel()
 			
 			if err := srv.Shutdown(shutdownCtx); err != nil {