Move the PinnedDevice viewset and discovery API residue into kolibri/core/discovery/viewsets/

[rtibbles]

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Overview

Move PinnedDeviceViewSet — migrated to the serializer-derived pattern in #14292 — from kolibri/core/discovery/api.py into kolibri/core/discovery/viewsets/pinned_device.py, with PinnedDeviceSerializer inline. As the last issue touching the discovery app's API modules, it also moves the remaining non-migrating code into the package and deletes api.py and serializers.py, per the File Organization convention in #14036.

Complexity: Low
Target branch: develop

Context

PinnedDeviceViewSet lives in kolibri/core/discovery/api.py with PinnedDeviceSerializer in serializers.py; its migration to the serializer-derived pattern lands via #14292 (PR #14818), so this is a pure relocation with no serialization changes.

The rest of the discovery API is residue that moves as-is: NetworkLocationViewSet and its Dynamic/Static subclasses (with NetworkLocationSerializer), and NetworkLocationFacilitiesView (with its inline _RemoteFacilityPicturePasswordSerializer/_RemoteFacilitySerializer) — per #14036, the latter never uses the ValuesViewset serialization path (it makes live HTTP requests).

The Change

Create the kolibri/core/discovery/viewsets/ package (empty __init__.py) and move PinnedDeviceViewSet with PinnedDeviceSerializer inline into kolibri/core/discovery/viewsets/pinned_device.py, and the network-location residue (viewsets, serializers, and NetworkLocationFacilitiesView) into one module per domain. Land each relocation as a pure-move commit, separate from import updates. Update imports (api_urls.py, tests, cross-references), then delete api.py and serializers.py.

Moving code breaks GitHub blame (local git blame -C -C still works), so digest the history the move obscures:

1. Run blame over the code being moved; flag hunks whose purpose isn't evident from the code or tests (odd guards, magic orderings, special cases).
2. Chase each flagged hunk to its originating PR/issue; where the rationale is non-obvious and load-bearing, carry it forward as a code comment citing the reference.
3. Where the rationale turns out to be obsolete, flag the code for removal — but as a pure move, this PR should not remove it; raise it for follow-up instead.

Out of Scope

• Any change to endpoint behavior or response shape — this is a pure move
• The serializer-derivation migration itself — that lands via Migrate PinnedDeviceViewSet to serializer-derived values pattern #14292 (PR Migrate PinnedDeviceViewSet to serializer-derived field introspection #14818)
• Migrating the network-location viewsets to the serializer-derived pattern — they're plain ModelViewSets, and NetworkLocationFacilitiesView never uses the ValuesViewset serialization path
• Removing obsolete code found during blame digestion — flag for follow-up instead

Acceptance Criteria

• kolibri/core/discovery/viewsets/pinned_device.py contains PinnedDeviceViewSet with PinnedDeviceSerializer inline; the network-location residue lives in the package, one module per domain
• api.py and serializers.py are deleted, with all references updated (api_urls.py, tests)
• Each relocation lands as a pure-move commit, separate from import updates
• Existing API tests pass unchanged
• Benchmark comparison passes for PinnedDeviceViewSet (data hash match + no performance regression)
• Comments citing the originating PR/issue added where blame digestion found non-obvious, load-bearing rationale

Testing

• Run the benchmark for PinnedDeviceViewSet, baseline with the old dotted path, comparison with the new one:

  python integration_testing/scripts/viewset_serialization_benchmark.py \
      kolibri.core.discovery.api.PinnedDeviceViewSet \
      --inherit-kolibri-home -o pinned_device_baseline.json
  
  # ... move ...
  
  python integration_testing/scripts/viewset_serialization_benchmark.py \
      kolibri.core.discovery.viewsets.pinned_device.PinnedDeviceViewSet \
      --inherit-kolibri-home --compare pinned_device_baseline.json

• The comparison must pass; include the before/after output in the PR description.
• Benchmark data setup (run via kolibri manage shell before benchmarking):

  from kolibri.core.discovery.models import PinnedDevice
  import uuid
  
  for i in range(100):
      PinnedDevice.objects.get_or_create(
          instance_id=uuid.uuid4(),
      )

AI usage

Drafted with AI assistance (Claude Code) from an interactive design discussion, section by section, with review and final edits by @rtibbles. Module contents, serialization paths, and the benchmark setup (carried over from #14292) were verified against the codebase during drafting.