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/enhancer.go

@@ -7,6 +7,7 @@ import (
 	"path/filepath"
 	"strings"
 
+	"github.com/insertr/insertr/internal/config"
 	"github.com/insertr/insertr/internal/engine"
 )
 
@@ -17,13 +18,8 @@ type EnhancementConfig struct {
 	GenerateIDs      bool
 }
 
-// DiscoveryConfig configures element discovery
-type DiscoveryConfig struct {
-	Enabled    bool
-	Aggressive bool
-	Containers bool
-	Individual bool
-}
+// Type alias for backward compatibility
+type DiscoveryConfig = config.DiscoveryConfig
 
 // Enhancer combines discovery, ID generation, and content injection in unified pipeline
 type Enhancer struct {