D3.1 Extension models and user model rename

Mirrors issue D3.1 Extension models and user model rename from the DataTalksClub/community-base unification plan (docs/plan/phase-3.md). Depends on: C5.2a (done). Implements playbook P7 (swap a site to the shared user model), DTC part, steps 1 to 3.

Goal

Move site-only user fields into extension models and rename the user model so the DTC accounts schema matches the shared community_base.accounts field table, preparing the accounts adoption (D3.2) and the C3.7 identity donor compatibility checkpoint.

Steps (from the plan)

1. courses.LearnerProfile with the course-platform fields; accounts_ext.IdentityState with the identity reconciliation fields and the AccountIdentityAlias, AccountIdentityQuarantine, AccountReconciliationRun, CmpLearnerImportProgress models; data migrations; readers rewritten.
2. RenameModel("CustomUser", "User") and AlterModelTable("accounts_user").
3. Field reconciliation to the C3.1 field table (add missing AISL-origin fields with defaults).

Verification (from the plan)

• uv run pytest -q -> pass
• development login works after deploy
• row counts equal before and after (playbook P14)

Non-goals

• Adopting the shared community_base.accounts app (that is D3.2, after C5.3 and C3.7).
• The django_migrations cleanup, squashed-migration fake, and AUTH_USER_MODEL flip described in P7 steps 4 and 5.

Start with docs/plan/phase-3.md and the existing courses.LearnerProfile and accounts_ext.IdentityState models, then trace the named identity and reconciliation models and their readers. Run uv run pytest -q during the migration work. Done means the tests pass, development login works after deployment, and row counts match before and after.