Fix unbounded activities lookup in listAirQoActive method

[coderabbitai]

Problem

The activities lookup in the listAirQoActive method in src/device-registry/models/Site.js (around lines 976-982) is currently unbounded and can cause memory issues and exploded response sizes.

Current Implementation

.lookup({
  from: "activities",
  localField: "_id",
  foreignField: "site_id",
  as: "activities",
})

Proposed Solution

Mirror the guarded lookup pattern used in the list() method by:

1. Adding maxActivities parsing and capping at the start of listAirQoActive
2. Replacing the simple lookup with a pipeline-based lookup that includes:
   • $match on site_id
   • $sort by createdAt descending
   • $limit using maxActivities
   • $project to only include needed fields

Benefits

• Prevents memory exhaustion from large activity datasets
• Maintains consistent behavior with the list() method
• Allows configurable activity limits while providing sensible defaults

References

• Original PR: enhance the response details of devices and sites #5170
• Code Review Comment: enhance the response details of devices and sites #5170 (comment)

Requested by

@Baalmart