Issue #3607392: Rename the Assignment plugin type to Audience

"Assignment" was too specific: the plugin type resolves both a task audience (candidates) and a notification audience (recipients), and account-less notification plugins (Email) had to be marked and filtered out of the task editor. This splits the concept and renames the type.

• AudienceInterface (recipients(), notifies()) is now the discovered plugin type. AssignmentInterface extends it and adds candidates()/viewerTokens() for the subset that also staffs user tasks.
• AudienceBase holds the shared notify flag, recipient wrapping and list normalizer; AssignmentBase extends it. Email / Email by variable are pure audiences, so NotificationOnlyAssignmentInterface is deleted and the task assignment editor now offers only AssignmentInterface plugins.
• Rename Attribute\Assignment -> Attribute\Audience, AssignmentManager -> AudienceManager, service orchestra.assignment_manager -> orchestra.audience_manager, namespace Plugin\Assignment -> Plugin\Audience, schema prefix orchestra.assignment.* -> orchestra.audience.*.
• Introduce RecipientInterface as the recipient value-object contract.

Stored workflow config is unaffected (plugin ids and node keys unchanged); verified 19 live workflows validate against the new schema. Follow-up to #3607367.