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/html_test.go

@@ -1,4 +1,3 @@
-// Package exporters_test provides tests for the html exporter.
 package exporters
 
 import (
@@ -270,7 +269,7 @@ func TestHTMLExporter_ProcessMultimediaItem(t *testing.T) {
 				Title: "<p>Video Title</p>",
 				Media: &models.Media{
 					Video: &models.VideoMedia{
-						OriginalUrl: "https://example.com/video.mp4",
+						OriginalURL: "https://example.com/video.mp4",
 						Duration:    120,
 					},
 				},
@@ -315,7 +314,7 @@ func TestHTMLExporter_ProcessImageItem(t *testing.T) {
 			{
 				Media: &models.Media{
 					Image: &models.ImageMedia{
-						OriginalUrl: "https://example.com/image.png",
+						OriginalURL: "https://example.com/image.png",
 					},
 				},
 				Caption: "<p>Image caption</p>",