Implement better file cache management with size limits and expiratio…

[Titi211179]

Fixes #34

This PR adds cache management to the downloaded file cache:

• Size limit: configurable maximum cache size (default 10 GB),
  removing least recently accessed files when exceeded
• Expiration: files older than a configurable period (default 30 days)
  are automatically re-downloaded
• JSON index: tracks download date, last access time and file size
  for each cached file
• Automatic cleanup: runs before each download, removing expired
  files and enforcing size limits
• Empty directory cleanup: removes leftover empty subdirectories

Settings are configurable via environment variables:

• CACHE_MAX_SIZE_GB (default: 10)
• CACHE_FILE_EXPIRY_DAYS (default: 30)