Adds support for local minio

[rgachuhi]

Summary

This PR adds local MinIO support to course-digest so media uploads can target a local S3-compatible store
during development.

What changed

• Added a --localMinio CLI option for upload flows.
• Added MinIO-specific environment handling for local development:
  • MINIO_ROOT_USER
  • MINIO_ROOT_PASSWORD
  • MINIO_BUCKET_NAME
  • MINIO_ENDPOINT
  • MINIO_MEDIA_URL_PREFIX
• Updated local MinIO mode to use MinIO-specific variables instead of falling back to AWS credentials.
• Added default local media URL prefix derivation when --localMinio is enabled and MINIO_MEDIA_URL_PREFIX
  is not set.
• Added support for stripping bucket-prefixed local MinIO URLs when deriving object keys for upload.
• Added .env.example entries documenting the new MinIO configuration.
• Updated the README with local MinIO setup and usage instructions.

Implementation details

• src/index.ts

  • added --localMinio
  • added helpers to derive the local MinIO bucket name and media URL prefix from env
  • routed upload actions through MinIO-aware config when the flag is enabled

• src/utils/upload.ts

  • added MinIO-aware upload config resolution
  • configured path-style S3 access for MinIO
  • ensured local MinIO uploads use MinIO env vars only
  • normalized object key extraction for local bucket-prefixed URLs

• .env.example

  • added MinIO example variables for local development

• README.md

  • documented .env-based local MinIO setup and usage

Tests

Added coverage for:

• local MinIO URL key extraction
• MinIO default config resolution
• MinIO upload client configuration
• local media URL prefix derivation from env
• ensuring local MinIO mode ignores AWS credentials