fix(fileserver): use streaming for video/audio to prevent memory exhaustion

- Add serveMediaStream() to stream video/audio without loading into memory
- Use http.ServeFile for local files (zero-copy, handles range requests)
- Redirect to S3 presigned URLs for S3-stored media files
- Refactor for better maintainability:
  - Extract constants and pre-compile lookup maps
  - Consolidate duplicated S3 client creation logic
  - Split authentication into focused helper methods
  - Group code by responsibility with section comments
  - Add setSecurityHeaders() and setMediaHeaders() helpers