feat(backend): adopt climate_ref.results.Reader for metric values

[lewisjared]

Summary

Migrates the backend's two /values endpoints off hand-rolled SQLAlchemy queries and onto the new climate_ref.results.Reader read-layer facade — the consumer that read layer was explicitly designed for. This is the first, keystone slice of a phased adoption (plan tracked separately); it targets the biggest duplication: metric-value filtering, source-id-aware IQR outlier detection, facet collection, and the load-all-then-paginate rule now all live upstream.

• GET /executions/{group_id}/values and GET /diagnostics/{provider}/{diagnostic}/values now query app_context.reader.values.{scalar,series}_values(...) via MetricValueFilter + OutlierPolicy.
• New get_reader dependency on AppContext (additive; the raw session dependency is untouched and still used by the not-yet-migrated routes).
• New core/reader_values.py for CSV rendering from reader collections and CV-dimension query-param gating; MetricValueCollection.build_{scalar,series}_from_reader adapters in models.py.

Response shapes are unchanged. promoted_only=False and include_retracted=True are set explicitly to preserve the previous unfiltered behaviour rather than adopt the reader's stricter defaults — so this is behaviour-preserving, which the existing route tests confirm.

Dependency / environment changes (please review)

• climate-ref is pinned to unreleased main (rev=67f1580) because the read layer is merged but not yet in a tagged release. Same pattern as the earlier metric-value-contract pin; swap to a tagged release (>=0.16) once one is published.
• Python floor moves 3.11 → 3.12, because climate-ref 0.15 requires >=3.12.
• The decimated test-fixture DB is migrated to the 0.15 schema (it was 4 alembic revisions behind; diagnostic.promoted_version was missing). This was done with climate-ref's own migrations — the same path a deployed 0.13-era database will take. Deployed REF databases must be alembic-migrated to head before a reader-based backend can read them.

Testing

• Full backend suite green: 185 passed, 1 xfailed (unchanged from the pre-migration baseline), against the git-pinned climate-ref.
• mypy (strict) and ruff clean.

Follow-ups (later phases)

• Migrate the executions/diagnostics list routes, datasets, and artifacts endpoints.
• Delete the now-dead core/outliers.py and the query half of core/metric_values.py once nothing references them.
• Harden the scalar adapter against null-valued scalars (float(item.value) would raise; none in the current fixture).

[netlify]

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Review details

⚙️ Run configuration

Configuration used: Organization UI

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Run ID: edf41615-7b59-4168-9519-e1dbe9c67258

📥 Commits

Reviewing files that changed from the base of the PR and between e501f1d and ca09520.

⛔ Files ignored due to path filters (2)

• backend/tests/test-data/tests.integration.test_cmip7_aft/test_solve_cmip7_aft/db/climate_ref.db is excluded by !**/*.db
• backend/uv.lock is excluded by !**/*.lock

📒 Files selected for processing (8)

• backend/pyproject.toml
• backend/src/ref_backend/api/deps.py
• backend/src/ref_backend/api/routes/diagnostics.py
• backend/src/ref_backend/api/routes/executions.py
• backend/src/ref_backend/core/reader_values.py
• backend/src/ref_backend/models.py
• changelog/39.breaking.md
• changelog/39.trivial.md

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/adopt-results-reader

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