Split databases and Redis — prepare infrastructure for per-domain isolation

Redis: per-app DB index (0-5) with shared auth DB 15 for SSO keys;
flushdb replaces flushall so deploys don't wipe cross-app auth state.

Postgres: drop 13 cross-domain FK constraints (migration v2t0p8q9r0),
remove dead ORM relationships, add explicit joins for 4 live ones.
Multi-engine sessions (account + federation) ready for per-domain DBs
via DATABASE_URL_ACCOUNT / DATABASE_URL_FEDERATION env vars.

All URLs initially point to the same appdb — zero behaviour change
until split-databases.sh is run to migrate data to per-domain DBs.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

shared/models/market.py

@@ -112,18 +112,8 @@ class Product(Base):
         back_populates="product",
         cascade="all, delete-orphan",
     )
-    cart_items: Mapped[List["CartItem"]] = relationship(
-        "CartItem",
-        back_populates="product",
-        cascade="all, delete-orphan",
-    )
-
-    # NEW: all order items that reference this product
-    order_items: Mapped[List["OrderItem"]] = relationship(
-        "OrderItem",
-        back_populates="product",
-        cascade="all, delete-orphan",
-    )
+    # cart_items and order_items live in a separate domain (cart DB)
+    # — cross-domain relationships removed
 
 from sqlalchemy import Column
 
@@ -131,7 +121,7 @@ class ProductLike(Base):
     __tablename__ = "product_likes"
 
     id = Column(Integer, primary_key=True, autoincrement=True)
-    user_id = Column(Integer, ForeignKey("users.id", ondelete="CASCADE"), nullable=False)
+    user_id = Column(Integer, nullable=False)
     product_slug: Mapped[str] = mapped_column(ForeignKey("products.slug", ondelete="CASCADE"))
 
     created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), nullable=False, server_default=func.now())
@@ -139,8 +129,6 @@ class ProductLike(Base):
     deleted_at: Mapped[Optional[datetime]] = mapped_column(DateTime(timezone=True))
     product: Mapped["Product"] = relationship("Product", back_populates="likes", foreign_keys=[product_slug])
 
-    user = relationship("User", back_populates="liked_products")  # optional, if you want reverse access
-
 
 class ProductImage(Base):
     __tablename__ = "product_images"
@@ -381,7 +369,7 @@ class CartItem(Base):
 
     # Either a logged-in user OR an anonymous session
     user_id: Mapped[int | None] = mapped_column(
-        ForeignKey("users.id", ondelete="CASCADE"),
+        Integer,
         nullable=True,
     )
     session_id: Mapped[str | None] = mapped_column(
@@ -389,9 +377,8 @@ class CartItem(Base):
         nullable=True,
     )
 
-    # IMPORTANT: link to product *id*, not slug
     product_id: Mapped[int] = mapped_column(
-        ForeignKey("products.id", ondelete="CASCADE"),
+        Integer,
         nullable=False,
     )
 
@@ -413,7 +400,7 @@ class CartItem(Base):
         server_default=func.now(),
     )
     market_place_id: Mapped[int | None] = mapped_column(
-        ForeignKey("market_places.id", ondelete="SET NULL"),
+        Integer,
         nullable=True,
         index=True,
     )
@@ -423,17 +410,19 @@ class CartItem(Base):
         nullable=True,
     )
 
-    # Relationships
-
+    # Cross-domain relationships — explicit join, viewonly (no FK constraint)
     market_place: Mapped["MarketPlace | None"] = relationship(
         "MarketPlace",
-        foreign_keys=[market_place_id],
+        primaryjoin="CartItem.market_place_id == MarketPlace.id",
+        foreign_keys="[CartItem.market_place_id]",
+        viewonly=True,
     )
     product: Mapped["Product"] = relationship(
         "Product",
-        back_populates="cart_items",
+        primaryjoin="CartItem.product_id == Product.id",
+        foreign_keys="[CartItem.product_id]",
+        viewonly=True,
     )
-    user: Mapped["User | None"] = relationship("User", back_populates="cart_items")
 
     __table_args__ = (
         Index("ix_cart_items_user_product", "user_id", "product_id"),