Submit a decision for a pending task.

Validates that option_id matches one of the task's options, persists
the decision, marks the task completed, and signals the waiting
Temporal workflow via complete_task.

The task must belong to the authenticated traveler (cross-traveler
submissions return 404). Submitting a decision for a task that is
already in a terminal state returns 409.

If the Temporal cluster is not configured, the decision is still
persisted and a 201 is returned with a warning log entry. If the
workflow has already completed/expired on the Temporal side, the decision
is persisted and 201 is also returned (workflow-gone is informational).