feat(data-analysis): create immutable deterministic Data Briefs

[1012839419a-alt]

Parent contract: #140
Program: #130
Depends on: #143 acceptance gate

Dependency

Start only after slice B (#143) supplies governed records and evidence refs.

Owned paths

• backend/schemas/data_artifact.py
• backend/models/data_artifact.py
• backend/migrations/versions/m0n1o2p3q4r5_add_data_brief_artifacts.py
• backend/plugins/data_analysis/manifest.py
• backend/plugins/data_analysis/service.py
• tests/unit/test_data_analysis_plugin.py

Required behavior

Implement analysis.brief.create as the reference-enabled but kernel-optional Data Analysis Plugin. Accept only the approved deterministic finite operations: schema/profile; completeness/duplicate/type quality; extraction/extractive summary; grouping/comparison/stable sorting; time trends; simple rule anomalies. Persist immutable Data Brief Artifact, Artifact Manifest and Claim Ledger with full Project/Run/Observation/Record/version/hash lineage. Unsupported claims are not findings.

Acceptance gate

Canonical input plus pinned operation/plugin versions reproduces the same Artifact hash. Missing plugin returns capability_unavailable without fallback. Tests cover idempotent replay, quality operations, bounded outputs, unsupported-claim exclusion, redaction, cross-Project denial and immutable hash integrity before slice D starts.