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/interfaces/html_cleaner.go

@@ -0,0 +1,13 @@
+package interfaces
+
+// HTMLCleaner defines the interface for cleaning HTML content.
+// Implementations convert HTML to plain text, stripping tags and decoding entities.
+type HTMLCleaner interface {
+	// CleanHTML removes HTML tags and decodes HTML entities from a string.
+	// Returns plain text suitable for non-HTML output formats.
+	CleanHTML(htmlStr string) string
+
+	// CleanAndTrim removes HTML tags, decodes entities, and trims whitespace.
+	// Convenience method combining CleanHTML with strings.TrimSpace.
+	CleanAndTrim(htmlStr string) string
+}