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/market.py

@@ -186,11 +186,18 @@ class NavTop(Base):
     id: Mapped[int] = mapped_column(primary_key=True)
     label: Mapped[str] = mapped_column(String(255), nullable=False)
     slug: Mapped[str] = mapped_column(String(255), nullable=False, index=True)
+    market_id: Mapped[Optional[int]] = mapped_column(
+        Integer,
+        ForeignKey("market_places.id", ondelete="SET NULL"),
+        nullable=True,
+        index=True,
+    )
     created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), nullable=False, server_default=func.now())
     updated_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), nullable=False, server_default=func.now())
     deleted_at: Mapped[Optional[datetime]] = mapped_column(DateTime(timezone=True))
-    
+
     listings: Mapped[List["Listing"]] = relationship(back_populates="top", cascade="all, delete-orphan")
+    market = relationship("MarketPlace", back_populates="nav_tops")
 
     __table_args__ = (UniqueConstraint("label", "slug", name="uq_nav_tops_label_slug"),)