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/services/html_cleaner.go

@@ -7,13 +7,19 @@ import (
 	"strings"
 
 	"golang.org/x/net/html"
+
+	"github.com/kjanat/articulate-parser/internal/interfaces"
 )
 
 // HTMLCleaner provides utilities for converting HTML content to plain text.
 // It removes HTML tags while preserving their content and converts HTML entities
 // to their plain text equivalents using proper HTML parsing instead of regex.
+// Implements interfaces.HTMLCleaner.
 type HTMLCleaner struct{}
 
+// Compile-time check that HTMLCleaner implements interfaces.HTMLCleaner.
+var _ interfaces.HTMLCleaner = (*HTMLCleaner)(nil)
+
 // NewHTMLCleaner creates a new HTML cleaner instance.
 // This service is typically injected into exporters that need to handle
 // HTML content from Articulate Rise courses.
@@ -46,6 +52,13 @@ func (h *HTMLCleaner) CleanHTML(htmlStr string) string {
 	return strings.TrimSpace(cleaned)
 }
 
+// CleanAndTrim removes HTML tags, decodes entities, and trims whitespace.
+// Convenience method that combines CleanHTML with strings.TrimSpace.
+// Returns empty string if input is empty or contains only whitespace/tags.
+func (h *HTMLCleaner) CleanAndTrim(htmlStr string) string {
+	return strings.TrimSpace(h.CleanHTML(htmlStr))
+}
+
 // extractText recursively traverses the HTML node tree and extracts text content.
 // It skips script and style tags to avoid including their content in the output.
 func extractText(w io.Writer, n *html.Node) {