feat: initial shared library extraction

Contains shared infrastructure for all coop services:
- shared/ (factory, urls, user_loader, context, internal_api, jinja_setup)
- models/ (User, Order, Calendar, Ticket, Product, Ghost CMS)
- db/ (SQLAlchemy async session, base)
- suma_browser/app/ (csrf, middleware, errors, authz, redis_cacher, payments, filters, utils)
- suma_browser/templates/ (shared base layouts, macros, error pages)
- static/ (CSS, JS, fonts, images)
- alembic/ (database migrations)
- config/ (app-config.yaml)
- editor/ (Lexical editor Node.js build)
- requirements.txt

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

models/cart_item.py

@@ -0,0 +1,70 @@
+from __future__ import annotations
+
+from datetime import datetime
+
+from sqlalchemy import Integer, String, DateTime, ForeignKey, func, Index
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+
+from db.base import Base  # you already import Base in app.py
+# from .user import User   # only if you normally import it here
+# from .coop import Product  # if not already in this module
+
+from .market import Product
+
+from .user import User
+
+class CartItem(Base):
+    __tablename__ = "cart_items"
+
+    id: Mapped[int] = mapped_column(Integer, primary_key=True, autoincrement=True)
+
+    # Either a logged-in user OR an anonymous session
+    user_id: Mapped[int | None] = mapped_column(
+        ForeignKey("users.id", ondelete="CASCADE"),
+        nullable=True,
+    )
+    session_id: Mapped[str | None] = mapped_column(
+        String(128),
+        nullable=True,
+    )
+
+    # IMPORTANT: link to product *id*, not slug
+    product_id: Mapped[int] = mapped_column(
+        ForeignKey("products.id", ondelete="CASCADE"),
+        nullable=False,
+    )
+
+    quantity: Mapped[int] = mapped_column(
+        Integer,
+        nullable=False,
+        default=1,
+        server_default="1",
+    )
+
+    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[datetime | None] = mapped_column(
+        DateTime(timezone=True),
+        nullable=True,
+    )
+
+    # Relationships
+   
+    product: Mapped["Product"] = relationship(
+        "Product",
+        back_populates="cart_items",
+    )
+    user: Mapped["User | None"] = relationship("User", back_populates="cart_items")
+
+    __table_args__ = (
+        Index("ix_cart_items_user_product", "user_id", "product_id"),
+        Index("ix_cart_items_session_product", "session_id", "product_id"),
+    )