perf/refactor: pre-compute device_categories at write time to eliminate runtime aggregation overhead

Summary

Currently, device_categories (including all_categories, primary_category, ownership_category, mobile_category, is_mobile, is_static) is computed at read time via a MongoDB $addFields aggregation stage (getDeviceCategoriesAddFieldsStage) every time a device document is queried. This stage runs inline on every call to statics.list() and every pipeline that fetches device data.

This issue tracks replacing that read-time computation with a write-time pre-computation approach: compute device_categories once when a device is created or updated, store it directly on the Device document, and replace the aggregation stage with a simple $project (or just read the stored field directly).

Background

device_categories is derived entirely from three stable device fields:

Field | Source
-- | --
category | Device document root
deployment_type | Device document root ("static" \| "mobile")
network | Device document root

Affected Files

• src/device-registry/models/Device.js — add stored field + pre-save hook + recomputeCategories static


• src/device-registry/utils/device.util.js — deprecate / remove getDeviceCategoriesAddFieldsStage()


• src/device-registry/models/Event.js — remove promotionStageForEventPipeline + addFields stage from fetch()


• src/device-registry/models/Reading.js — remove $ifNull passthrough; read stored field directly


• src/device-registry/jobs/store-readings-job.js — replace passthrough with Device lookup read


• Migration script — one-time recomputeCategories({}) backfill

Out of Scope

• Changing the shape of DeviceCategorySchema (no field additions/removals in this issue)


• Unit tests for computeDeviceCategories() — tracked separately in #5XXX


• The enrich-dev-cats PR currently in review — this issue is a follow-up optimisation after that PR lands

Priority

Low–Medium. The correctness fix (Event.js promotion stage + cron recompute) in the enrich-dev-cats PR resolves the immediate corruption bug. This issue is a performance and maintainability improvement for a subsequent PR.

Labels: performance, refactor, device-registry

Component: device-registry

Related PRs: enrich-dev-cats branch (must land first)