Restructures the public interest endpoint to create the yacht as a
first-class row (owned by the new client, or by a newly upserted
company when a company block is provided) and writes the yacht_id
onto the new interest. All writes now run inside a single
transaction instead of the previous unwrapped sequence.
The public validator gains structured `yacht` (required) and
`company` (optional) sub-objects; legacy flat fields remain in the
schema for backward compatibility but are silently ignored.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
