Simpler data from connected systems: whoever can write, writes

Per-field ownership rules are gone. A connected source, such as a PLM or an ERP, writes the fields it sends, and the last write wins. Nothing is locked in the product editor: anyone who can edit a product — and Atomo, imports and the API — can change any field.

Access is now just read or write: give a system a read-only Catalog API key if it should never change your catalog. Every change shows up in the product's history with its author, and can be restored. Each connection shows what the source sent and the fields the catalog doesn't know.

For existing Catalog API integrations: keys that were set to never overwrite ("Nucleo" rule) are now read-only; keys set to fill only empty fields ("insert" rule) now overwrite existing values; `GET /ping` no longer returns `key.field_rule`; the `409 field_rule` and `409 field_locked` errors no longer exist.