Native multipart/form-data deserialization with blob support

## Request

Add native support for deserializing `multipart/form-data` so form uploads can route directly to Harper resources. File parts should be translated to blobs automatically.

## Design considerations

- Streaming: form data should stream into the resource API rather than buffer the entire body first (opt-in per resource class)
- Blob assignment: blobs cannot receive data until assigned to a table; deserialization currently runs before the resource/table call. Options: buffer files to a temp dir until assignment, or allow the table to signal its readiness before deserialization completes

🤖 Filed by Claude on behalf of Kris.