Scraper can't fetch seller profile age without following each listing's
seller link. Using 0 as sentinel caused every scraped seller to trigger
new_account and account_under_30_days red flags erroneously.
- Seller.account_age_days: int → Optional[int] (None = not yet fetched)
- Migration 003: recreate sellers table without NOT NULL constraint
- MetadataScorer: return None for unknown age → score_is_partial=True
- Aggregator: gate age flags on is not None
- Scraper: account_age_days=None instead of 0
