Upload file content for a pragma/file resource.
The file is stored in GCS and can be referenced by a pragma/file resource. After uploading, apply the resource YAML to make it available:
provider: pragma
resource: file
name: my-file
config:
content_type: application/pdf
lifecycle_state: pending
Returns: FileMetadata with URL, size, checksum, and upload timestamp.
Raises: HTTPException: 413 if file exceeds maximum size.