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_test.go

@@ -1,4 +1,3 @@
-// Package exporters_test provides tests for the docx exporter.
 package exporters
 
 import (
@@ -331,7 +330,7 @@ func TestDocxExporter_ComplexCourse(t *testing.T) {
 									Caption: "<p>Watch this introductory video</p>",
 									Media: &models.Media{
 										Video: &models.VideoMedia{
-											OriginalUrl: "https://example.com/intro.mp4",
+											OriginalURL: "https://example.com/intro.mp4",
 											Duration:    300,
 										},
 									},
@@ -359,7 +358,7 @@ func TestDocxExporter_ComplexCourse(t *testing.T) {
 									Caption: "<p>Course overview diagram</p>",
 									Media: &models.Media{
 										Image: &models.ImageMedia{
-											OriginalUrl: "https://example.com/overview.png",
+											OriginalURL: "https://example.com/overview.png",
 										},
 									},
 								},