Add comprehensive godoc documentation to all exported symbols

- Document all exported types, functions, methods, and constants across 52 files
- Add package-level doc comments to client, models, utils, and ynison packages
- Document HTTP method and endpoint path for each API client method
- Document model struct fields and DeJSON/DeList/ToJSON/ToMap methods
- Fix duplicate package doc comment in internal/models/base.go

Co-Authored-By: Claude Code <noreply@anthropic.com>

internal/client/request.go

@@ -13,7 +13,7 @@ import (
 	"gitea.mrixs.me/Mrixs/yandex-music-api-go/internal/utils"
 )
 
-// Default values
+// Default values for the Yandex Music API client.
 const (
 	DefaultBaseURL      = "https://api.music.yandex.net"
 	DefaultTimeout      = 5 * time.Second
@@ -21,7 +21,7 @@ const (
 	DefaultClientHeader = "YandexMusicAndroid/24023621"
 )
 
-// RequestConfig holds configuration for the request client
+// RequestConfig holds configuration for the request client.
 type RequestConfig struct {
 	BaseURL             string
 	Token               string
@@ -36,14 +36,16 @@ type RequestConfig struct {
 	OnSchemaMismatch    utils.SchemaMismatchHandler
 }
 
-// Request handles HTTP requests to Yandex Music API
+// Request handles HTTP requests to the Yandex Music API.
+// It manages the HTTP client, authentication headers, and request signing.
 type Request struct {
 	config        RequestConfig
 	client        *http.Client
 	schemaTracker *utils.SchemaMismatchTracker
 }
 
-// NewRequest creates a new Request instance
+// NewRequest creates a new Request instance with the given configuration.
+// It sets up the HTTP client, default headers, and authentication if a token is provided.
 func NewRequest(config RequestConfig) *Request {
 	if config.BaseURL == "" {
 		config.BaseURL = DefaultBaseURL
@@ -92,7 +94,8 @@ func NewRequest(config RequestConfig) *Request {
 	return r
 }
 
-// SetToken updates the authorization token
+// SetToken updates the authorization token used for API requests.
+// When a token is provided, it sets the Authorization header to "OAuth <token>".
 func (r *Request) SetToken(token string) {
 	r.config.Token = token
 	if token != "" {
@@ -102,33 +105,38 @@ func (r *Request) SetToken(token string) {
 	}
 }
 
-// SetLanguage sets the Accept-Language header
+// SetLanguage sets the Accept-Language header for API requests.
 func (r *Request) SetLanguage(lang string) {
 	r.config.Language = lang
 	r.config.Headers["Accept-Language"] = lang
 }
 
-// Get performs a GET request
+// Get performs a GET request to the given path with query parameters.
+// The response is unmarshaled into the result pointer.
 func (r *Request) Get(ctx context.Context, path string, params map[string]string, result any) error {
 	return r.doRequest(ctx, "GET", path, params, nil, result)
 }
 
-// Post performs a POST request
+// Post performs a POST request to the given path with a JSON body.
+// The response is unmarshaled into the result pointer.
 func (r *Request) Post(ctx context.Context, path string, body any, result any) error {
 	return r.doRequest(ctx, "POST", path, nil, body, result)
 }
 
-// Put performs a PUT request
+// Put performs a PUT request to the given path with a JSON body.
+// The response is unmarshaled into the result pointer.
 func (r *Request) Put(ctx context.Context, path string, body any, result any) error {
 	return r.doRequest(ctx, "PUT", path, nil, body, result)
 }
 
-// Delete performs a DELETE request
+// Delete performs a DELETE request to the given path with an optional JSON body.
+// The response is unmarshaled into the result pointer.
 func (r *Request) Delete(ctx context.Context, path string, body any, result any) error {
 	return r.doRequest(ctx, "DELETE", path, nil, body, result)
 }
 
-// doRequest performs the actual HTTP request
+// doRequest performs the actual HTTP request to the Yandex Music API.
+// It handles URL building, request signing, response parsing, and error handling.
 func (r *Request) doRequest(ctx context.Context, method, path string, params map[string]string, body any, result any) error {
 	// Build URL
 	fullURL := r.config.BaseURL + path
@@ -209,7 +217,7 @@ func (r *Request) doRequest(ctx context.Context, method, path string, params map
 	return nil
 }
 
-// handleErrorResponse handles HTTP error responses
+// handleErrorResponse handles HTTP error responses by mapping status codes to typed errors.
 func (r *Request) handleErrorResponse(statusCode int, body []byte) error {
 	var msg string
 	apiResp, err := utils.DeJSON(body)
@@ -235,7 +243,8 @@ func (r *Request) handleErrorResponse(statusCode int, body []byte) error {
 	}
 }
 
-// Download downloads a file from a URL
+// Download downloads a file from the given URL and returns the raw response body.
+// It is used for downloading track files and lyrics text.
 func (r *Request) Download(ctx context.Context, urlStr string) ([]byte, error) {
 	req, err := http.NewRequestWithContext(ctx, "GET", urlStr, nil)
 	if err != nil {