A project must stay tied to one customer. The quote/contract/hours attach
paths already rejected a foreign customer (equality on project.customer_account_id);
the two remaining holes are closed here:
- assignEvent: an event may only join a project that shares its customer. The
event's customer(s) come from event_customer_assignments; a customer-assigned
project rejects an event for a different customer (PROJECT_CUSTOMER_MISMATCH),
and an empty project ADOPTS a single-customer event's customer. This is why
a foreign-customer event could previously be attached.
- updateProject: re-labelling a project to a customer that conflicts with the
events/quotes/contracts it already holds is rejected (clearing to null is
still allowed), so the customer can't be swapped out from under existing
content.
Frontend: the cockpit attach-event action surfaces the translated mismatch
message; projects.error.customerMismatch reworded to read for both documents
and events (de + en).