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.

.github/workflows/ci.yml

@@ -467,7 +467,7 @@ jobs:
           # Process a local file (mount current directory)
           docker run --rm -v $(pwd):/workspace ghcr.io/${{ github.repository }}:latest /workspace/input.json markdown /workspace/output.md
           ```
-          
+
           EOF
 
   # Security and quality analysis workflows