Enforce soft-deleted entities are unhooked

[stephenh]

And in general ergonomics like:

• A `onSoftDeleteCascade("children") for a soft-deleted to soft-delete its owned children
• A `onSoftDeleteUnset("projectItem") for a soft-deleted entity to unhook itself from other entities

---

Examples

Cascade to soft-deleted children & on soft-delete cascade a hard deletion to other children:

/** Cascade cascade soft deletes to dependencies. */
// TODO(soft-deleted): https://github.com/stephenh/joist-ts/issues/910
config.beforeFlush(["requiredTaskDocuments", "scheduleTaskChecklistItems"], async (task, { em }) => {
  if (task.isSoftDeletedEntity && task.changes.deletedAt.hasChanged) {
    const dependencies = (
      await Promise.all([task.successorDependencies.load(), task.predecessorDependencies.load()])
    ).flat();
    dependencies.forEach((d) => d.softDelete());
    task.requiredTaskDocuments.get.forEach((rtd) => em.delete(rtd));
    task.scheduleTaskChecklistItems.get.forEach((stci) => em.delete(stci));
  }
});

/** On task soft-delete, unhook any project items. */
// TODO(soft-deleted): https://github.com/stephenh/joist-ts/issues/910
config.beforeFlush(async (task) => {
  if (task.isSoftDeletedEntity) {
    const projectItems = await task.load((t) => t.projectItems);
    projectItems.forEach((pi) => pi.task.set(undefined));
  }
});

Maybe prevent soft deletion?

/** Tasks on a draw schedule, or a global milestone, cannot be soft-deleted. */
config.addRule(async (task) => {
  if (task.isSoftDeletedEntity) {
    const draws = await task.load((t) => t.homeownerContractDraws);
    if (draws.length > 0) {
      return "Cannot delete tasks on a draw schedule";
    }
    if (!task.allowMilestoneDelete && (await task.isGlobalMilestoneTask())) {
      return "Milestone tasks cannot be deleted";
    }
  }
});