D3.2 Freeze weekend: adopt shared accounts and onboarding

Plan issue D3.2, phase 3 of the community-base unification plan.

Spec: docs/plan/phase-3.md, section D3.2, in DataTalksClub/community-base. Status is tracked in docs/plan/STATUS.md there, which stays the source of truth.

Depends on: C5.3, C3.7, D3.1. Do not start until every one of them is done in STATUS.md.

Freeze issue. Playbook P13 applies: this moves data during a scheduled weekend freeze of the site. Do not start it outside a planned freeze window.

Process

This repository's AGENTS.md and _docs/PROCESS.md govern this work, with _docs/specs/ as the product authority. The plan says what to build and how to verify it; the site process says how work is done here.

Spec

Repository: DataTalksClub/website. Depends on: C5.3, C3.7, D3.1. Freeze required: yes. Playbook P7 step 4, P13.

Steps

1. Delete local accounts app code except accounts_ext; install community_base.accounts,
   questionnaires, onboarding, community, notifications, comments; migrate accounts 0001_squashed --fake
   after deleting DTC's accounts rows from django_migrations.
2. Configure flows: default flow (profile, questionnaire "Welcome"); a learners group flow
   (profile, questionnaire "Course goals"). ONBOARDING_ELIGIBILITY: verified email.
3. Slack grant on onboarding completion; reveal page; the public /slack landing stays.
4. Enable comments on event and course pages, notifications for event and cohort changes.

Production checks (development environment, DTC is not at the apex yet)

• register, verify, complete the default flow, open /accounts/community/slack/ -> 200 with the
  invite; a user in learners sees the second questionnaire.

Done when

• spec 01 "Member profile version 1" marked implemented by the package; spec 05 Slack section updated

Read docs/plan/phase-3.md section D3.2, docs/plan/STATUS.md, AGENTS.md, _docs/PROCESS.md, and the relevant _docs/specs/ before starting; first confirm C5.3, C3.7, and D3.1 are marked done and a freeze window is planned. Done means the shared accounts and onboarding flows, Slack invite, comments, and notifications work as specified, with the listed production checks passing and specs 01 and 05 updated.