Batch production settings, descriptor copy at creation and assignee update events

[frankrousseau]

Problem

• Attaching task types, task statuses and asset types to a production takes one request per item: production creation and the import-from-production flows fire dozens of requests (the task type import even relies on a client-side timer between removals and additions).
• Creating a production requires the web client to copy every all-projects metadata descriptor onto it, one create request per descriptor, and API-created projects miss those columns entirely.
• Updating a task's assignee list in one PUT works but emits no task:assign/task:unassign events and creates no assignation notification, so it cannot replace the one-request-per-person assign flow.

Solution

• Add POST /data/projects//settings/batch: attaches task types (with priorities), task statuses and asset types in one request, skips unknown ids, and supports replace_task_types so an import expresses the full wanted set in a single call.
• Copy Project-scoped metadata descriptors from open projects onto new projects at creation, server-side.
• Emit task:assign/task:unassign and create assignation notifications from the task update resource when the assignees list changes.