Remove internal/content package and use engine directly

- Eliminate content.Enhancer wrapper around engine.ContentEngine
- Update cmd/enhance.go to call engine.ProcessFile/ProcessDirectory directly
- Update sites/manager.go to use engine.NewContentEngineWithAuth directly
- Remove unused EnhancementConfig and discovery configuration logic
- Simplify enhance command to use input path as site ID
- Remove CLI config dependency from config package

internal/config/validation.go

@@ -5,6 +5,7 @@ import (
 	"net/url"
 	"os"
 	"path/filepath"
+	"slices"
 	"strings"
 )
 
@@ -17,10 +18,6 @@ func validate(config *Config) error {
 		return fmt.Errorf("api config: %w", err)
 	}
 
-	if err := validateCLI(config.CLI); err != nil {
-		return fmt.Errorf("cli config: %w", err)
-	}
-
 	if err := validateServer(config.Server); err != nil {
 		return fmt.Errorf("server config: %w", err)
 	}
@@ -66,18 +63,6 @@ func validateAPI(config APIConfig) error {
 	return nil
 }
 
-func validateCLI(config CLIConfig) error {
-	if config.SiteID == "" {
-		return fmt.Errorf("site_id is required")
-	}
-
-	if strings.TrimSpace(config.SiteID) != config.SiteID {
-		return fmt.Errorf("site_id cannot have leading or trailing whitespace")
-	}
-
-	return nil
-}
-
 func validateServer(config ServerConfig) error {
 	if config.Port < 1 || config.Port > 65535 {
 		return fmt.Errorf("port must be between 1 and 65535, got %d", config.Port)
@@ -136,13 +121,7 @@ func validateAuth(config AuthConfig) error {
 	}
 
 	validProviders := []string{"mock", "authentik"}
-	valid := false
-	for _, provider := range validProviders {
-		if config.Provider == provider {
-			valid = true
-			break
-		}
-	}
+	valid := slices.Contains(validProviders, config.Provider)
 	if !valid {
 		return fmt.Errorf("invalid provider %q, must be one of: %s", config.Provider, strings.Join(validProviders, ", "))
 	}