fix: reject incomplete JW schedule chunks

[tiankaima]

Goal

Prevent a partial JW schedule refresh from being cached or published as a complete static snapshot.

Closes #16.

Changes

• Derive the expected JW chunk count from Catalog lessons using the existing 100-course contract.
• Record jw_schedule_chunk_size and per-semester expected chunk metadata without changing schema v5.
• Require successful, unique, contiguous chunk indexes 0..N-1 before reusing a semester cache.
• Re-raise non-JSON JW failures so CI cannot publish the partial build.
• Preserve compatibility with older v5 snapshots by deriving the expected count from stored Catalog rows when metadata is absent.

Evidence

• uv run python -m unittest discover -s tests -p 'test_*.py' — 30 passed.
• uv run ruff check src tests — passed.
• Added focused complete, missing-chunk, failed-chunk, duplicate-safe cache coverage.

Risk

A failed refresh can leave partial rows in that failed job's local SQLite file, but the workflow exits unsuccessfully and does not upload or publish it; the next job restores the last successful artifact.