Document file storage: Serverpod Cloud, S3/MinIO, local dev, and upload progress

[developerjamiu]

Demand

Kapa "File Upload & Storage" cluster — 9 conversations / 8 users. (The profile-photo image flow is covered by PR #643.)

Problem

12-file-uploads.md covers the database (default) backend and external providers, but:

• Serverpod Cloud storage configuration is missing or unclear.
• S3-compatible / self-hosted MinIO setup is not covered, although users asked directly ("does Serverpod do s3?", "how to use a local minio storage?").
• No local-development walkthrough, and no guidance on switching backends (dev vs production).
• The upload-progress API is not documented.
• No recommended field type for referencing a stored file in a model.

Scope

• Serverpod Cloud storage configuration.
• S3-compatible / MinIO configuration.
• Local-dev uploads and switching backends (dev vs prod).
• Upload progress (or document the limitation), and the recommended field type to reference a stored file.

Verify before writing

• Whether MinIO / custom S3 endpoints are supported; whether FileUploader exposes progress (confirm against source).