feat(storage): configure upload http metadata

[ClaudiaFang]

Feature

Allow configuring HTTP metadata when uploading files to S3-compatible storage, including Cloudflare R2.

Requirements

• Set Content-Type correctly for uploaded files based on their actual media type or extension (for example, image/webp, image/jpeg, image/png).
• Add a configurable Cache-Control value for uploaded objects.
• Apply the metadata consistently to S3-compatible uploads, including Cloudflare R2.
• Preserve sensible defaults so existing users do not need to change their configuration.
• Document the new settings and recommended values for immutable/versioned assets versus replaceable files.

Acceptance criteria

• WebP uploads are served as image/webp, not image/jpeg.
• The configured cache policy is sent with the upload request and is visible in the object response headers.
• Existing upload workflows remain compatible when the new settings are omitted.
• Tests cover MIME type detection and metadata propagation.

[ClaudiaFang]

Backlog priority review — 2026-08-04

Priority: P1

Reason: Incorrect Content-Type is a real output-correctness defect, and missing cache metadata affects serving behavior and performance. The scope and tests are well defined, with backward-compatible defaults available.

Scheduling note: Implement after #19/PR #20 is settled to avoid settings conflicts. Separate automatic MIME correctness from optional advanced cache-policy configuration if needed.