Issue #3611190: Record the process instance initiator (start-user)

Records a first-class initiator (start-user) on the process instance, so a requester-facing surface can answer "which instances did this user start?".

• New initiator base field on orchestra_instance: a plain uid, empty for a system/API/cron start; stored as a bare id (not a user entity reference), matching the work item assignee, so the runtime entity keeps no hard dependency on the user module.
• Set at start via a new optional argument to ProcessControlInterface::start(); a subprocess inherits its parent's; never defaulted to the current user, so a programmatic start is not mis-attributed.
• Indexed, so "the instances a user initiated" is a paged, index-served query (no load-all finder ships: the set is unbounded).
• Aligns with Camunda startUserId / camunda:initiator and the WS-HumanTask task initiator (distinct from a task's actual owner, which orchestra models as the assignee).

Kernel test, docs (concepts.md) and French translations included. Foundation for follow-ups: the webform handler populating the initiator from the submission owner, and a requester "my requests" surface.