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/order.py

@@ -14,11 +14,11 @@ class Order(Base):
 
     id: Mapped[int] = mapped_column(Integer, primary_key=True, autoincrement=True)
 
-    user_id: Mapped[Optional[int]] = mapped_column(ForeignKey("users.id"), nullable=True)
+    user_id: Mapped[Optional[int]] = mapped_column(Integer, nullable=True)
     session_id: Mapped[Optional[str]] = mapped_column(String(64), index=True, nullable=True)
 
     page_config_id: Mapped[Optional[int]] = mapped_column(
-        ForeignKey("page_configs.id", ondelete="SET NULL"),
+        Integer,
         nullable=True,
         index=True,
     )
@@ -69,9 +69,12 @@ class Order(Base):
         cascade="all, delete-orphan",
         lazy="selectin",
     )
+    # Cross-domain relationship — explicit join, viewonly (no FK constraint)
     page_config: Mapped[Optional["PageConfig"]] = relationship(
         "PageConfig",
-        foreign_keys=[page_config_id],
+        primaryjoin="Order.page_config_id == PageConfig.id",
+        foreign_keys="[Order.page_config_id]",
+        viewonly=True,
         lazy="selectin",
     )
 
@@ -86,7 +89,7 @@ class OrderItem(Base):
     )
 
     product_id: Mapped[int] = mapped_column(
-        ForeignKey("products.id"),
+        Integer,
         nullable=False,
     )
     product_title: Mapped[Optional[str]] = mapped_column(String(512), nullable=True)
@@ -106,9 +109,11 @@ class OrderItem(Base):
         back_populates="items",
     )
 
-    # NEW: link each order item to its product
+    # Cross-domain relationship — explicit join, viewonly (no FK constraint)
     product: Mapped["Product"] = relationship(
         "Product",
-        back_populates="order_items",
+        primaryjoin="OrderItem.product_id == Product.id",
+        foreign_keys="[OrderItem.product_id]",
+        viewonly=True,
         lazy="selectin",
     )