perf(api): eliminate N+1 queries, add query guards and missing indexes

- Notification fan-out: replace sequential for loops with Promise.all (allocation-effects, notification-broadcast, create-notification)
- Public holiday batch: group resources by location combo, resolve holidays once per group, replace per-holiday delete/findFirst/create with 3 batched queries (~18K → ~5 queries)
- Add take guards to unbounded findMany calls (resource-analytics: 5000, resource-marketplace: 2000, resource-capacity: 1000, chargeability-report: 2000)
- auto-staffing: add select with only needed fields + take: 5000
- schema.prisma: add 5 missing indexes (ManagementLevel.groupId, Blueprint.isActive/target, Comment.parentId, Vacation.requestedById, Resource.managementLevelGroupId)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/api/src/lib/create-notification.ts

@@ -93,10 +93,7 @@ export async function createNotificationsForUsers(
   params: Omit<CreateNotificationParams, "userId"> & { userIds: string[] },
 ): Promise<number> {
   const { userIds, ...rest } = params;
-  let count = 0;
-  for (const userId of userIds) {
-    await createNotification({ ...rest, userId });
-    count++;
-  }
-  return count;
+  if (userIds.length === 0) return 0;
+  await Promise.all(userIds.map((userId) => createNotification({ ...rest, userId })));
+  return userIds.length;
 }