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/interfaces/exporter.go

@@ -1,5 +1,3 @@
-// Package interfaces provides the core contracts for the articulate-parser application.
-// It defines interfaces for parsing and exporting Articulate Rise courses.
 package interfaces
 
 import "github.com/kjanat/articulate-parser/internal/models"