Add two envs to Mediaflow and always sign with the public one

[syntaxsdev]

Details:

• PUBLIC_S3_ENDPOINT = what clients hit (e.g., http://localhost:9000 or http://:9000)

• INTERNAL_S3_ENDPOINT = what servers/workers use (e.g., http://minio:9000)

• Ensure path-style is on for MinIO.