Add durable pipeline status reporting

[anth-volk]

Fixes #962

Summary

• add a Modal pipeline status callable and protected HTTP endpoint
• persist durable redacted pipeline error records with canonical stage/substage IDs
• expose bounded status payloads with run manifests, stage manifests, missing expected manifests, and traceback context
• add AI-facing pipeline operations guidance for diagnosing deployed failures

Tests

• make lint
• uv run --no-sync --with pytest pytest tests/unit/test_pipeline_status.py tests/unit/test_step_manifest.py tests/unit/test_pipeline.py tests/unit/test_pipeline_source_contracts.py
• PYTHONPATH=. /opt/miniconda3/bin/pytest tests/unit/test_pipeline.py
• PYTHONPATH=. /opt/miniconda3/bin/pytest tests/integration/test_modal_pipeline_seams.py (skips without Modal credentials)
• uv run --no-sync --with pyyaml python scripts/run_quality_guards.py
• uv run --no-sync --with ruff ruff check modal_app/step_manifests/status.py modal_app/step_manifests/errors.py policyengine_us_data/utils/error_redaction.py policyengine_us_data/utils/step_manifest.py modal_app/pipeline.py tests/unit/test_pipeline_status.py tests/unit/test_pipeline.py tests/integration/test_modal_pipeline_seams.py
• git diff --check