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/auth/auth.go

@@ -13,6 +13,7 @@ import (
 
 	"github.com/coreos/go-oidc/v3/oidc"
 	"github.com/golang-jwt/jwt/v5"
+	"github.com/insertr/insertr/internal/config"
 	"golang.org/x/oauth2"
 )
 
@@ -24,31 +25,10 @@ type UserInfo struct {
 	Provider string `json:"iss,omitempty"`
 }
 
-// AuthConfig holds authentication configuration
-type AuthConfig struct {
-	DevMode      bool
-	Provider     string
-	JWTSecret    string
-	OAuthConfigs map[string]OAuthConfig
-	OIDC         *OIDCConfig
-}
-
-// OAuthConfig holds OAuth provider configuration
-type OAuthConfig struct {
-	ClientID     string
-	ClientSecret string
-	RedirectURL  string
-	Scopes       []string
-}
-
-// OIDCConfig holds OIDC configuration for Authentik
-type OIDCConfig struct {
-	Endpoint     string
-	ClientID     string
-	ClientSecret string
-	RedirectURL  string
-	Scopes       []string
-}
+// Type aliases for backward compatibility
+type AuthConfig = config.AuthConfig
+type OAuthConfig = config.OAuthConfig
+type OIDCConfig = config.OIDCConfig
 
 // AuthService handles authentication operations
 type AuthService struct {