Introduces per-page markets (Phase 2):
- MarketPlace model with soft-delete and unique slug index
- NavTop gains market_id FK to scope nav hierarchy per market
- Migration with backfill creates default 'suma-market' for existing market page
- market_url_for() cross-app URL helper
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
