Move remaining coach plugin API code into kolibri/plugins/coach/viewsets/

[rtibbles]

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Overview

Move the coach plugin's remaining API code — api.py, class_summary_api.py, unit_report_api.py, unit_lesson_progress_api.py, and serializers.py — into a kolibri/plugins/coach/viewsets/ package, one module per domain, deleting the old modules. This is the cleanup issue for the coach plugin under the File Organization convention in #14036; it covers everything except the classroom notifications domain, which moves with its migration in #14311.

Complexity: Medium
Target branch: develop

Context

The coach plugin already deviates from the single-module pattern — its API code spans four modules plus serializers.py:

• api.py: LessonReportViewset (serializer in serializers.py), the difficult-questions family (BaseExerciseDifficultQuestionsViewset, ExerciseDifficultQuestionsViewset, QuizDifficultQuestionsViewset, PracticeQuizDifficultQuestionsViewset), and the classroom notifications domain (moving in Migrate ClassroomNotificationsViewset to serializer-derived values pattern #14311)
• class_summary_api.py: ClassSummaryViewSet plus its serialization helper functions
• unit_report_api.py: UnitReportViewSet plus test-score computation helpers
• unit_lesson_progress_api.py: UnitLessonProgressViewSet

Apart from ClassroomNotificationsViewset, none of these use the ValuesViewset serialization path, so this is a pure move with no serialization changes. Permission classes, filters, and module-level helpers move with their viewsets. A natural split is lesson_report.py, difficult_questions.py, class_summary.py, unit_report.py, and unit_lesson_progress.py; the final split is decided in the PR.

The Change

Move the remaining coach API code into kolibri/plugins/coach/viewsets/, one module per domain, with LessonReportSerializer inline above LessonReportViewset. Update imports in api_urls.py and anywhere else that references the moved classes, then delete api.py, class_summary_api.py, unit_report_api.py, unit_lesson_progress_api.py, and serializers.py. Land each module's relocation as a pure-move commit, separate from import updates.

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.

How to Get There

All affected endpoints are routed through the router in kolibri/plugins/coach/api_urls.py (lessonreport, classsummary, the difficult-questions routes, unitreport, etc.) — the single routing module whose imports this move updates.

Out of Scope

• Any change to endpoint behavior or response shape — this is a pure move
• The classroom notifications domain (ClassroomNotificationsViewset, its filter and permissions) — moves with its migration in Migrate ClassroomNotificationsViewset to serializer-derived values pattern #14311
• Migrating any coach viewset to the serializer-derived pattern — none of the moved viewsets use the ValuesViewset serialization path
• Removing obsolete code found during blame digestion — flag for follow-up instead

Acceptance Criteria

• kolibri/plugins/coach/viewsets/ contains all remaining coach API viewsets, permissions, filters, and helper functions, one module per domain, with LessonReportSerializer inline above LessonReportViewset
• api.py, class_summary_api.py, unit_report_api.py, unit_lesson_progress_api.py, and serializers.py are deleted
• All references updated (api_urls.py, tests, any cross-module imports)
• Each relocation lands as a pure-move commit, separate from import updates
• Existing API tests pass unchanged
• Comments citing the originating PR/issue added where blame digestion found non-obvious, load-bearing rationale

Testing

• pytest kolibri/plugins/coach/test/ must pass unchanged — test_class_summary.py, test_difficult_questions.py, test_lesson_report.py, test_unit_report.py, and test_unit_lesson_progress.py cover all moved endpoints.

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, routing, and test coverage were verified against the codebase during drafting.