refactor: Align with Go conventions and improve maintainability

Renames the `OriginalUrl` field to `OriginalURL` across media models to adhere to Go's common initialisms convention. The `json` tag is unchanged to maintain API compatibility.

Introduces constants for exporter formats (e.g., `FormatMarkdown`, `FormatDocx`) to eliminate the use of magic strings, enhancing type safety and making the code easier to maintain.

Additionally, this commit includes several minor code quality improvements:
- Wraps file-writing errors in exporters to provide more context.
- Removes redundant package-level comments from test files.
- Applies various minor linting fixes throughout the codebase.

internal/exporters/docx.go

@@ -69,7 +69,7 @@ func (e *DocxExporter) Export(course *models.Course, outputPath string) error {
 
 	// Ensure output directory exists and add .docx extension
 	if !strings.HasSuffix(strings.ToLower(outputPath), ".docx") {
-		outputPath = outputPath + ".docx"
+		outputPath += ".docx"
 	}
 
 	// Create the file
@@ -198,5 +198,5 @@ func (e *DocxExporter) exportSubItem(doc *docx.Docx, subItem *models.SubItem) {
 // Returns:
 //   - A string representing the supported format ("docx")
 func (e *DocxExporter) SupportedFormat() string {
-	return "docx"
+	return FormatDocx
 }