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/content/site_manager.go

@@ -8,18 +8,12 @@ import (
 	"strings"
 	"sync"
 
+	"github.com/insertr/insertr/internal/config"
 	"github.com/insertr/insertr/internal/engine"
 )
 
-// SiteConfig represents configuration for a registered site
-type SiteConfig struct {
-	SiteID      string           `yaml:"site_id"`
-	Path        string           `yaml:"path"`        // Served path (enhanced output)
-	SourcePath  string           `yaml:"source_path"` // Source path (for enhancement)
-	Domain      string           `yaml:"domain,omitempty"`
-	AutoEnhance bool             `yaml:"auto_enhance"`
-	Discovery   *DiscoveryConfig `yaml:"discovery,omitempty"` // Override discovery settings
-}
+// Type alias for backward compatibility
+type SiteConfig = config.SiteConfig
 
 // SiteManager handles registration and enhancement of static sites
 type SiteManager struct {