fix(ci): harden weekly ETL schedule

[Sam-24-dev]

Summary

• move the weekly ETL schedule to Monday 08:17 UTC to avoid the top-of-hour scheduler hotspot
• add a pre-publication canonical source freshness guard using dataset_summaries.updated_at_utc
• document controlled recovery of delayed runs through workflow_dispatch

Validation

• python -m pytest -q (220 passed)
• python -m pytest -q tests/test_check_source_freshness.py tests/test_workflow_etl_contract.py (11 passed)
• python scripts/check_source_freshness.py --project-root . --max-source-age-hours 192
• static YAML parse and assertions for schedule, workflow_dispatch, and concurrency
• detect-secrets scan of staged files

Scope

No ETL, deployment, source datasets, frontend, dependencies, or permissions changes.

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