fix: support S3-compatible services (MinIO)

[applearound]

I deploy Minio for self host s3 service, but is seems unsupportted.

Uploading files to self-hosted S3-compatible services (e.g. MinIO) fails with:

body must be a string, ArrayBuffer or ArrayBufferView, unless you include the X-Amz-Content-Sha256 header.

Root cause: aws4fetch cannot parse service and region from non-AWS endpoints (like localhost), which causes:

1. The automatic X-Amz-Content-Sha256: UNSIGNED-PAYLOAD header (only set when service === 's3') is not applied
2. ReadableStream body fails to sign because aws4fetch can't hash it
3. X-Amz-Credential in the Authorization header contains wrong service, rejected by MinIO

Additionally, S3_REGION was declared in docker-compose.example.yml but never read by the code.

• Add service:'s3' to AwsClient to fix ReadableStream signing
• Add S3_PROTOCOL config (default: https)
• Add S3_TYPE config for virtual-hosted vs path-style URLs
• Read S3_REGION from env (was declared in docker-compose but unused)

No breaking changes for existing AWS S3 users — all new configs have defaults matching current behavior.