ART M6: approval and transactional staged import (BR-32, BR-33, BR-34; ADR-0002) #110

Plan: docs/art/milestone-0-assessment.md §5b. Approved 26 Oct.

• Submit, approve and reject. The uploader can't approve their own data by default (decision 5); a per-tenant setting allows it for single-user tenants.
• COPY into staging, then promote in one API-owned transaction, with an idempotency key. No partial imports (decision 4).
• Lineage, plus reconciliation of rows in, excluded, edited and imported.
• This also closes the post-commit audit gap in the README limitations.

Verified by: reject-then-resubmit, failure injected mid-import, retry, and concurrent import tests.

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