D0.1c Copy settings and switch readers and writers

Plan issue D0.1c, phase 0 of the community-base unification plan.

Spec: docs/plan/phase-0.md, section D0.1c, in DataTalksClub/community-base. Status is tracked in docs/plan/STATUS.md there, which stays the source of truth.

Depends on: D0.1b. Do not start until every one of them is done in STATUS.md.

Currently blocked in STATUS.md: merged as website 3fb9db07; dev deploy blocked on a fully green main CI run (website#345: the corpus-bound django test clusters are fixed, merged as website fc534fcf; the Playwright lane remains in flight); worker self-check IAM grant merged as aws-infra 3acefe43 but not yet effective: the protected development apply is blocked on the sandbox-account trust-policy bootstrap (aws-infra#56)

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: D0.1b.

Prerequisite: every recorded shared parity gap is resolved in a published package tag.

Steps

1. Install config/API and declare mapped keys. Add reversible historical-model data copy with
   explicit type, secret-encryption and audit-history mappings.
2. Preserve DTC all-or-nothing batch, revision, If-Match, idempotency and capability protections
   through adapters. Mount routes without shadowing /api/v1/admin or /api/v1/me.
3. Rehearse on a permitted development copy. Compare exact counts and non-secret invariants;
   second copy adds zero rows, malformed input causes zero partial writes.
4. Switch readers and writers together under reviewed deployment sequencing. Retain old storage
   read-only for rollback. Verify no divergent writes before reverse copy.

Verification

• Development-copy migration, reverse/reapply and exact row comparisons pass.
• Settings permission, batch, revision, source badge and redaction regression scenarios pass.
• Site versioned verification plan, independent tester/PM and development deploy gates pass.

Done when

• the deployed cutover is reconciled and rollback instructions are verified

Docs

• Settings mapping, deployment evidence and rollback runbook.

Start with docs/plan/phase-0.md section D0.1c and confirm the prerequisite status in docs/plan/STATUS.md before examining AGENTS.md, _docs/PROCESS.md, and _docs/specs/. Review the existing settings mapping and deployment evidence, then follow the development-copy migration and verification plan. Done means the deployed cutover is reconciled, required regression and parity checks pass, and rollback instructions are verified.