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/models/item_types.go

@@ -0,0 +1,41 @@
+package models
+
+// Item type constants used throughout the application.
+// These match the type field values in Articulate Rise course data.
+const (
+	// ItemTypeText represents text content blocks with headings and paragraphs.
+	ItemTypeText = "text"
+
+	// ItemTypeList represents bullet or numbered list content.
+	ItemTypeList = "list"
+
+	// ItemTypeKnowledgeCheck represents quiz or assessment items.
+	ItemTypeKnowledgeCheck = "knowledgecheck"
+
+	// ItemTypeMultimedia represents video or audio content.
+	ItemTypeMultimedia = "multimedia"
+
+	// ItemTypeImage represents standalone image content.
+	ItemTypeImage = "image"
+
+	// ItemTypeInteractive represents interactive elements like accordions or tabs.
+	ItemTypeInteractive = "interactive"
+
+	// ItemTypeDivider represents visual separator elements.
+	ItemTypeDivider = "divider"
+
+	// ItemTypeQuote represents quotation blocks.
+	ItemTypeQuote = "quote"
+
+	// ItemTypeFlipcard represents flashcard-style content.
+	ItemTypeFlipcard = "flipcard"
+)
+
+// Lesson type constants.
+const (
+	// LessonTypeSection represents a section header (not a real lesson).
+	LessonTypeSection = "section"
+
+	// LessonTypeLesson represents a regular lesson.
+	LessonTypeLesson = "lesson"
+)