CBE app foundation + CompetencyTaxonomy model #640

EDIT:

This subtask was mixed up accidentally - it was just a copy of #641 . I had Claude fix it; but I haven't reviewed this in-depth yet.
Either way the parent ticket can be viewed as source of truth.

Description

Parent: #613. This ticket is the foundation of the three-way split: it creates the Django
app the CBE models live in and the single model that both follow-up tickets depend on,
CompetencyTaxonomy. See #613 for full background, ADR-0001
for app placement, and ADR-0002/ADR-0003 for the model.

Nothing else can start until this merges: #641 needs the app to exist and needs
CompetencyTaxonomy for CompetencyRuleProfile.competency_taxonomy_id; #642 needs the app to
exist. Once this is in, #641 and #642 can be developed in parallel.

What to build

1. The openedx_learning umbrella app and the cbe applet

ADR-0001 places CBE at src/openedx_learning/applets/cbe/, inside a new top-level
openedx_learning app that aggregates its applets into one Django app and Python API, the same
way src/openedx_content does today. That umbrella package does not exist in this repo yet —
ADR-0001 assumed the Learning Pathways workstream would have created it, and it hasn't. So this
ticket creates it:

• src/openedx_learning/__init__.py, src/openedx_learning/apps.py — one AppConfig for the
  whole app (name/label = openedx_learning), mirroring ContentConfig in
  src/openedx_content/apps.py. Individual applets do not get their own apps.py.
• src/openedx_learning/applets/__init__.py
• src/openedx_learning/applets/cbe/__init__.py, models.py (or models/), admin.py
• src/openedx_learning/applets/cbe/migrations/0001_initial.py

2. Wire it up

• Add "openedx_learning" to INSTALLED_APPS in test_settings.py.
• Add openedx_learning to root_packages in .importlinter, and place it in the
  src_layering contract above openedx_tagging. The constraint that matters (agreed in
  [BE] Implement CBE core data models (CompetencyTaxonomy, criteria, learner status) #613's comments and consistent with [BE] Update taxonomy Create & Import endpoints to accept Taxonomy Type #614): the CBE applet may depend on openedx_tagging;
  openedx_tagging must never depend on or know about CBE.

django-simple-history needs no requirements work: django-simple-history==3.12.0 is already in
requirements/base.txt. #613's Technical Notes say "add if not already present" — it is present.

3. CompetencyTaxonomy

Per ADR-0002 Decision 1, a Django multi-table-inheritance subclass of openedx_tagging's
Taxonomy, marking a taxonomy as CBE-enabled:

class CompetencyTaxonomy(Taxonomy):
    ...

• taxonomy_ptr_id is both the PK and the one-to-one FK to oel_tagging_taxonomy.id. It is
  created implicitly by MTI — do not declare it manually.
• Not a taxonomy_type column on oel_tagging_taxonomy. The ADR rejected that.
• No HistoricalRecords(). ADR-0003 Decision 2 explicitly excludes CompetencyTaxonomy,
  oel_tagging_taxonomy, and oel_tagging_tag from django-simple-history.

Acceptance criteria (from #613)

Copied verbatim from #613; only the ones this ticket is responsible for.

• CompetencyTaxonomy uses Django MTI (not a taxonomy_type column); taxonomy_ptr_id is both the PK and FK to oel_tagging_taxonomy.id
• django-simple-history is not applied to oel_tagging_tag, oel_tagging_taxonomy, or CompetencyTaxonomy
• No columns exist on any model that are not defined in the ADRs
• All FK relationships match the ADR definitions exactly

Additional acceptance criteria (from splitting this out of #613)

Not in #613 verbatim; needed because #613 was split into three tickets.

• The app exists at src/openedx_learning/applets/cbe/ per ADR-0001, with a single
  AppConfig for openedx_learning, and is registered in test_settings.py
INSTALLED_APPS.
• .importlinter is updated: openedx_learning added to root_packages and placed in
  src_layering above openedx_tagging. lint-imports passes. The rules are not loosened
  to make it pass.
• migrations/0001_initial.py creates the CompetencyTaxonomy table and applies cleanly
  against an empty database. ([BE] Implement CBE core data models (CompetencyTaxonomy, criteria, learner status) #613's "apply cleanly from scratch" is verified end-to-end
  across the three merged tickets.)
• CompetencyTaxonomy is annotated .. no_pii:. This is this ticket's slice of [BE] Implement CBE core data models (CompetencyTaxonomy, criteria, learner status) #613's
  PII-annotation AC; the full make pii_check 100%-coverage gate is verified in Mastery status lookup + learner progress models #642.
• No index from ADR-0002 Decision 5 lands in this ticket: none of them target
  CompetencyTaxonomy. Called out so the whole-feature "all indexes present" gate (verified
  in Mastery status lookup + learner progress models #642) isn't read as a miss here.

Open questions to confirm before starting

1. Does this ticket really create the openedx_learning umbrella app? ADR-0001 defers that
   to the Learning Pathways workstream, which has not started and has no date. The alternative,
   putting CBE under openedx_content/applets/, was explicitly rejected by ADR-0001 (Alt 3)
   because it would couple the authoring-only openedx_content app to AUTH_USER_MODEL and
   runtime learner-status concerns. Creating the umbrella here is the ADR-compliant reading, but
   it is more work than "add an applet" and should be confirmed.
2. taxonomy_overrides_org. The current ADR-0002 on branch jesperhodge/competency-adr-4
   adds a taxonomy_overrides_org boolean (default false) to CompetencyTaxonomy, used only
   as the org-vs-taxonomy tiebreak in Decision 4, and read by no code path in this phase. It did
   not exist in the ADR revision this ticket family was written against, and ADR-0004/ADR-0005
   are not yet on main. Confirm whether this column is in scope now or lands when that ADR PR
   merges.

Out of scope

The three criteria models CompetencyCriteriaGroup, CompetencyCriteria,
CompetencyRuleProfile (#641); the mastery-status lookup and learner progress models (#642);
delete protection and on_delete=PROTECT (#642); any history tracking; any REST API or UI work.

Depends on

Nothing. This is the first of the three to merge and unblocks both #641 and #642.