Create a StoryPlan and enqueue its async generation.

Validates target + artifactRequests + sourceIds synchronously, including source ownership and status READY, persists the plan as PENDING, and enqueues the planner backed by DB sidecar story_api_plan. Requires Idempotency-Key for safe retries. Refused with 409 project_precondition_failed until the project passes its full Series Bible coverage — details.stage names the stage to finish; retrying will not help. Non-blocking warning gate: before a screenplay/table_read plan, read or generate the full project Series Bible and review the plan against it. The planner auto-loads the saved Series Bible as canonical project truth. The response includes a preliminary quote computed from the artifact requests; the worker overwrites it with a plan-aware quote once the StoryPlan envelope is generated. Table-read plans accept creativeBrief, return a generated scriptBlueprint, cannot use autoApprove, and are limited to one table_read script/installment per plan. Table-read artifact requests may set narrationPolicy (auto | include | suppress) to make narrator/no-narrator a user-level creation choice instead of implying it from mode.