Refactor configuration system with centralized type-safe config package

- Create internal/config package with unified config structs and validation
- Abstract viper dependency behind config.Loader interface for better testability
- Replace manual config parsing and type assertions with type-safe loading
- Consolidate AuthConfig, SiteConfig, and DiscoveryConfig into single package
- Add comprehensive validation with clear error messages
- Remove ~200 lines of duplicate config handling code
- Maintain backward compatibility with existing config files

internal/config/loader.go

@@ -0,0 +1,106 @@
+package config
+
+import (
+	"fmt"
+
+	"github.com/spf13/viper"
+)
+
+type Loader interface {
+	Load(configFile string) (*Config, error)
+	LoadWithDefaults() (*Config, error)
+	LoadWithFlags(dbPath, apiURL, apiKey, siteID string) (*Config, error)
+}
+
+type viperLoader struct{}
+
+func NewLoader() Loader {
+	return &viperLoader{}
+}
+
+func (l *viperLoader) Load(configFile string) (*Config, error) {
+	v := viper.New()
+
+	if configFile != "" {
+		v.SetConfigFile(configFile)
+	} else {
+		v.AddConfigPath(".")
+		v.SetConfigName("insertr")
+		v.SetConfigType("yaml")
+	}
+
+	v.SetEnvPrefix("INSERTR")
+	v.AutomaticEnv()
+
+	if err := v.ReadInConfig(); err != nil {
+		if _, ok := err.(viper.ConfigFileNotFoundError); !ok {
+			return nil, fmt.Errorf("failed to read config file: %w", err)
+		}
+	}
+
+	config := &Config{}
+	if err := v.Unmarshal(config); err != nil {
+		return nil, fmt.Errorf("failed to unmarshal config: %w", err)
+	}
+
+	if err := l.setDefaults(config); err != nil {
+		return nil, err
+	}
+
+	return config, validate(config)
+}
+
+func (l *viperLoader) LoadWithDefaults() (*Config, error) {
+	return l.Load("")
+}
+
+func (l *viperLoader) LoadWithFlags(dbPath, apiURL, apiKey, siteID string) (*Config, error) {
+	config, err := l.LoadWithDefaults()
+	if err != nil {
+		return nil, err
+	}
+
+	if dbPath != "" {
+		config.Database.Path = dbPath
+	}
+	if apiURL != "" {
+		config.API.URL = apiURL
+	}
+	if apiKey != "" {
+		config.API.Key = apiKey
+	}
+	if siteID != "" {
+		config.CLI.SiteID = siteID
+	}
+
+	return config, validate(config)
+}
+
+func (l *viperLoader) setDefaults(config *Config) error {
+	if config.Database.Path == "" {
+		config.Database.Path = "./insertr.db"
+	}
+
+	if config.CLI.SiteID == "" {
+		config.CLI.SiteID = "demo"
+	}
+
+	if config.Server.Port == 0 {
+		config.Server.Port = 8080
+	}
+
+	if config.Server.Host == "" {
+		config.Server.Host = "localhost"
+	}
+
+	if config.Auth.Provider == "" {
+		config.Auth.Provider = "mock"
+	}
+
+	if config.Auth.JWTSecret == "" {
+		config.Auth.JWTSecret = "dev-secret-change-in-production"
+		config.Auth.DevMode = true
+	}
+
+	return nil
+}