Complete rename of all environment variable references from MINIO_
prefix to S3_ prefix across the entire codebase:
- internal/config/constants.go: EnvPrefix and all Env* constants
- All Go source files: string literals, error messages, comments
- Prometheus metrics: minio_* → s3_* metric name prefix
- Dockerfiles: ENV declarations
- Build scripts, CI workflows, helm charts, docs
- Grafana dashboards: metric queries updated
Go import paths and package names unchanged.
Build verified: go build ./cmd/... passes clean.