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)

go.mod

@@ -3,9 +3,21 @@ module DanceLessonsCoach
 go 1.26.1
 
 require (
+	github.com/fsnotify/fsnotify v1.9.0 // indirect
 	github.com/go-chi/chi/v5 v5.2.5 // indirect
+	github.com/go-viper/mapstructure/v2 v2.4.0 // indirect
 	github.com/mattn/go-colorable v0.1.14 // indirect
 	github.com/mattn/go-isatty v0.0.20 // indirect
+	github.com/pelletier/go-toml/v2 v2.2.4 // indirect
 	github.com/rs/zerolog v1.35.0 // indirect
+	github.com/sagikazarmark/locafero v0.11.0 // indirect
+	github.com/sourcegraph/conc v0.3.1-0.20240121214520-5f936abd7ae8 // indirect
+	github.com/spf13/afero v1.15.0 // indirect
+	github.com/spf13/cast v1.10.0 // indirect
+	github.com/spf13/pflag v1.0.10 // indirect
+	github.com/spf13/viper v1.21.0 // indirect
+	github.com/subosito/gotenv v1.6.0 // indirect
+	go.yaml.in/yaml/v3 v3.0.4 // indirect
 	golang.org/x/sys v0.42.0 // indirect
+	golang.org/x/text v0.28.0 // indirect
 )