Engine Setup Endpoint

Dispatch a branching-engine setup as an async operation.

The actual provisioning (Neon project, pgstream, snapshot, RLS) runs 5–10 minutes — far past the ALB 60s idle timeout — so this endpoint returns 202 with an operation_id within ~1s. The CLI polls GET /v1/operations/{id} for stage transitions and the terminal outcome (ARD-741).

Fast paths preserved from the prior synchronous endpoint:

• Already-healthy connector → 200 with the existing connector row.
• Wrong starting status → 400 with the rejected status name.
• Service has no engine-setup handler → 400. Anything past that point goes to the worker.

Requirements:

• Connector must exist
• branching_engine_status must be retryable for setup dispatch