[Media] Write unit tests for Media bounded context

[Shifrin]

Goal

Unit tests for everything in src/Media/.

Acceptance criteria

• MediaFile: upload, updateAltText, delete — events recorded correctly
• FilePath: rejects path traversal
• MimeType: allowed types, disallowed types
• FileSize: toHumanReadable() for bytes, KB, MB, GB
• Code coverage ≥ 90%
• PHPStan level 8 passes