feat: add MarketPlace model, migration, and market_url_for helper

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>

models/__init__.py

@@ -14,6 +14,7 @@ from .ghost_membership_entities import (
 
 from .calendars import Calendar, CalendarEntry, Ticket
 from .page_config import PageConfig
+from .market_place import MarketPlace