Issue #3611204: Surface the process instance initiator: list column, Views field and filters

Surfaces the process instance initiator (#3611190) so it is visible and filterable.

• orchestra_interaction_webform: a webform-started run records its submission owner as the initiator; anonymous submissions leave it unset.
• orchestra_ui: the instances list gains an Initiator column (the start-user, linked) and a filter by initiator through a user autocomplete.
• orchestra_views: an Initiator field (uid rendered as a linked username), the raw initiator filtered by core's username autocomplete (user_name), and an 'Initiated by the current user' filter for a my-instances list.

The initiator stays a bare uid on the instance (no user entity reference); only the display resolves the user, so orchestra core keeps no user dependency while orchestra_ui and orchestra_views declare one. Kernel tests, docs and French translations included. Follow-up: the 'My workflow instances' page and View.