feat: decouple cart from shared_lib, add app-owned models

Phase 1-3 of decoupling:
- path_setup.py adds project root to sys.path
- Cart-owned models in cart/models/ (order, page_config)
- All imports updated: shared.infrastructure, shared.db, shared.browser, etc.
- PageConfig uses container_type/container_id instead of post_id FK

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

models/__init__.py

@@ -0,0 +1,2 @@
+from .order import Order, OrderItem
+from .page_config import PageConfig

models/order.py

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+from __future__ import annotations
+
+from datetime import datetime
+from typing import Optional, List
+
+from sqlalchemy import Integer, String, DateTime, ForeignKey, Numeric, func, Text
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+
+from shared.db.base import Base
+
+
+class Order(Base):
+    __tablename__ = "orders"
+
+    id: Mapped[int] = mapped_column(Integer, primary_key=True, autoincrement=True)
+
+    user_id: Mapped[Optional[int]] = mapped_column(ForeignKey("users.id"), 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"),
+        nullable=True,
+        index=True,
+    )
+
+    status: Mapped[str] = mapped_column(
+        String(32),
+        nullable=False,
+        default="pending",
+        server_default="pending",
+    )
+    currency: Mapped[str] = mapped_column(String(16), nullable=False, default="GBP")
+    total_amount: Mapped[float] = mapped_column(Numeric(12, 2), nullable=False)
+
+    # free-form description for the order
+    description: Mapped[Optional[str]] = mapped_column(Text, nullable=True, index=True)
+
+    # SumUp reference string (what we send as checkout_reference)
+    sumup_reference: Mapped[Optional[str]] = mapped_column(
+        String(255),
+        nullable=True,
+        index=True,
+    )
+
+    # SumUp integration fields
+    sumup_checkout_id: Mapped[Optional[str]] = mapped_column(
+        String(128),
+        nullable=True,
+        index=True,
+    )
+    sumup_status: Mapped[Optional[str]] = mapped_column(String(32), nullable=True)
+    sumup_hosted_url: Mapped[Optional[str]] = mapped_column(Text, nullable=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(),
+        onupdate=func.now(),
+    )
+
+    items: Mapped[List["OrderItem"]] = relationship(
+        "OrderItem",
+        back_populates="order",
+        cascade="all, delete-orphan",
+        lazy="selectin",
+    )
+    calendar_entries: Mapped[List["CalendarEntry"]] = relationship(
+        "CalendarEntry",
+        back_populates="order",
+        lazy="selectin",
+    )
+    page_config: Mapped[Optional["PageConfig"]] = relationship(
+        "PageConfig",
+        foreign_keys=[page_config_id],
+        lazy="selectin",
+    )
+
+
+class OrderItem(Base):
+    __tablename__ = "order_items"
+
+    id: Mapped[int] = mapped_column(Integer, primary_key=True, autoincrement=True)
+    order_id: Mapped[int] = mapped_column(
+        ForeignKey("orders.id", ondelete="CASCADE"),
+        nullable=False,
+    )
+
+    product_id: Mapped[int] = mapped_column(
+        ForeignKey("products.id"),
+        nullable=False,
+    )
+    product_title: Mapped[Optional[str]] = mapped_column(String(512), nullable=True)
+
+    quantity: Mapped[int] = mapped_column(Integer, nullable=False, default=1)
+    unit_price: Mapped[float] = mapped_column(Numeric(12, 2), nullable=False)
+    currency: Mapped[str] = mapped_column(String(16), nullable=False, default="GBP")
+
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True),
+        nullable=False,
+        server_default=func.now(),
+    )
+
+    order: Mapped["Order"] = relationship(
+        "Order",
+        back_populates="items",
+    )
+
+    # NEW: link each order item to its product
+    product: Mapped["Product"] = relationship(
+        "Product",
+        back_populates="order_items",
+        lazy="selectin",
+    )

models/page_config.py

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+from __future__ import annotations
+
+from datetime import datetime
+from typing import Optional
+
+from sqlalchemy import Integer, String, Text, DateTime, func, JSON, text
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+
+from shared.db.base import Base
+
+
+class PageConfig(Base):
+    __tablename__ = "page_configs"
+
+    id: Mapped[int] = mapped_column(Integer, primary_key=True, autoincrement=True)
+
+    container_type: Mapped[str] = mapped_column(
+        String(32), nullable=False, server_default=text("'page'"),
+    )
+    container_id: Mapped[int] = mapped_column(Integer, nullable=False)
+
+    features: Mapped[dict] = mapped_column(
+        JSON, nullable=False, server_default="{}"
+    )
+
+    # Per-page SumUp credentials (NULL until configured)
+    sumup_merchant_code: Mapped[Optional[str]] = mapped_column(String(64), nullable=True)
+    sumup_api_key: Mapped[Optional[str]] = mapped_column(Text(), nullable=True)
+    sumup_checkout_prefix: Mapped[Optional[str]] = mapped_column(String(64), nullable=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), nullable=True
+    )