fix(compose): honor external S3 settings

[ShiroKSH]

Problem

The Compose relay environment hardcoded its S3 endpoint and addressing style, so values set in .env were silently ignored. That sent media to the bundled MinIO service instead of the configured S3-compatible provider.

Fixes #3794.

Change

• Preserve the bundled MinIO defaults when S3 variables are unset.
• Pass BUZZ_S3_ENDPOINT, BUZZ_S3_REGION, and BUZZ_S3_ADDRESSING_STYLE from .env to the relay.
• Document the external-storage path and that the base stack still starts MinIO.

Validation

• git diff --check
• Rendered Compose config with default values: MinIO endpoint, us-east-1, and path addressing.
• Rendered Compose config with external overrides: endpoint, region, and virtual addressing passed through unchanged.