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/ghost_content.py

@@ -172,6 +172,14 @@ class Post(Base):
         passive_deletes=True,
     )
 
+    markets: Mapped[List["MarketPlace"]] = relationship(
+        "MarketPlace",
+        back_populates="post",
+        cascade="all, delete-orphan",
+        passive_deletes=True,
+        order_by="MarketPlace.name",
+    )
+
 class Author(Base):
     __tablename__ = "authors"