Backfill Contract Term Seed

Create term 1 for every candidate contract, copying effective_date → start_date, expiration_date → end_date, and auto_renews straight through. Enriching a term with its notice date, notice period and renewal term from the contract-sourced annual_key_dates rows is a separate backfill, since that needs the key-date recurrence math.

Materializes no reminder rows (materialize_reminders=False): reminders start flowing at cutover, not at seed time, and a ladder per seeded term would be a mass email the next morning. Those terms get their ladder the first time anything touches them.

Each contract is written inside its own savepoint, so one whose stored dates violate the term CHECK is reported in skipped_invalid_dates instead of aborting the run.