refactor: extract HTMLCleaner interface and consolidate constants

- Add interfaces.HTMLCleaner interface for better testability
- Add CleanAndTrim() method to simplify repeated pattern
- Move item/lesson type constants to models/item_types.go
- Update all exporters to use interface instead of concrete type
- Replace strings.TrimSpace(CleanHTML(...)) with CleanAndTrim()

internal/exporters/docx.go

@@ -13,7 +13,6 @@ import (
 
 	"github.com/kjanat/articulate-parser/internal/interfaces"
 	"github.com/kjanat/articulate-parser/internal/models"
-	"github.com/kjanat/articulate-parser/internal/services"
 )
 
 // Font sizes for DOCX document headings (in half-points, so "32" = 16pt).
@@ -28,7 +27,7 @@ const (
 // using the go-docx package.
 type DocxExporter struct {
 	// htmlCleaner is used to convert HTML content to plain text
-	htmlCleaner *services.HTMLCleaner
+	htmlCleaner interfaces.HTMLCleaner
 	// logger is used for logging warnings and errors
 	logger interfaces.Logger
 }
@@ -42,7 +41,7 @@ type DocxExporter struct {
 //
 // Returns:
 //   - An implementation of the Exporter interface for DOCX format
-func NewDocxExporter(htmlCleaner *services.HTMLCleaner, logger interfaces.Logger) interfaces.Exporter {
+func NewDocxExporter(htmlCleaner interfaces.HTMLCleaner, logger interfaces.Logger) interfaces.Exporter {
 	return &DocxExporter{
 		htmlCleaner: htmlCleaner,
 		logger:      logger,