Manual graph apply backend split

[XucroYuri]

Summary

• Add Manual Merge/Split Canonical Graph design and execution plan.
• Add audited canonical graph operation log plus idempotent merge/split preview and apply methods.
• Expose protected research graph merge/split/operations APIs with 404/422 error mapping and focused tests.

Split boundary

• Kept this PR to backend/store/API plus tests.
• Deferred legacy target_workbench UI apply controls because current main does not carry the old target workbench route/files; UI should be reintroduced in a current-admin-shell split.

Verification

• PYTHONPATH=src /Volumes/SSD/Code/06-dev-tools/NewsSentry/.venv/bin/python -m pytest tests/unit/test_async_store.py -k 'canonical_graph or canonical_merge or canonical_split' -q --no-cov
• PYTHONPATH=src /Volumes/SSD/Code/06-dev-tools/NewsSentry/.venv/bin/python -m pytest tests/unit/test_api_server.py -k 'research_graph' -q --no-cov
• PYTHONPATH=src /Volumes/SSD/Code/06-dev-tools/NewsSentry/.venv/bin/python -m pytest tests/unit/test_async_store.py -q --no-cov
• PYTHONPATH=src /Volumes/SSD/Code/06-dev-tools/NewsSentry/.venv/bin/python -m pytest tests/unit/test_api_server.py -q --no-cov
• PYTHONPATH=src /Volumes/SSD/Code/06-dev-tools/NewsSentry/.venv/bin/python -m ruff check
• PYTHONPATH=src /Volumes/SSD/Code/06-dev-tools/NewsSentry/.venv/bin/python -m mypy src/news_sentry/ --ignore-missing-imports
• PYTHONPATH=src /Volumes/SSD/Code/06-dev-tools/NewsSentry/.venv/bin/python tools/scan_sensitive_data.py
• PYTHONPATH=src /Volumes/SSD/Code/06-dev-tools/NewsSentry/.venv/bin/python tools/check_no_hardcoded_target.py
• PYTHONPATH=src /Volumes/SSD/Code/06-dev-tools/NewsSentry/.venv/bin/python -m pytest tests/unit/test_config_schema_validation.py -q --no-cov
• PYTHONPATH=src uv run --python 3.12 --extra dev --extra api --extra proxy python -m pytest tests/ -q --tb=short --timeout=300 --cov=src/news_sentry --cov-report=term-missing --cov-report=xml

Note: the full pytest run passed with existing Starlette/httpx deprecation and aiosqlite event-loop warnings.

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Reject merges into retired canonical events

If survivor_canonical_event_id points to an event already marked status == "merged", this path still accepts it as the survivor and _apply_canonical_merge_plan later moves the candidate mentions onto that retired node while leaving its merged_into metadata intact. In the scenario where A was already merged into B, a subsequent manual merge of C into A will strand C's mentions under A instead of the active survivor B; add a survivor-specific status/merged_into check before building the merge plan.

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