Preserve scheduler audit evidence

[crypdick]

What changed

• Add bounded SQLite scheduler definitions and occurrence evidence with retention watermarks and audit classification.
• Record Temporal-scheduled agent, database-host, and configured host-cron outcomes using Temporal activity schedule timestamps.
• Add retention settings, operator documentation, and regression coverage for audit classification and pruning.

Verification

• uv run pytest -o addopts='' scheduler-focused suite: 204 passed.
• uv run ruff check src tests
• uv run mkdocs build --strict
• uvx prek run --all-files

Resolves SYN-33