Decouple cross-app models: move canonical definitions to shared/models/

- Move 6 model files to shared/models/ (ghost_content, order, page_config,
  market, market_place, calendars) so apps import from shared, not each other
- Fix auth templates: replace url_for('auth.*') with coop_url() for
  cross-app compatibility
- Fix TYPE_CHECKING import in sumup.py to use shared.models.order
- Delete dead infrastructure/cart_loader.py (inverted dependency)
- Update models/__init__.py to export all new models

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

models/market_place.py

@@ -0,0 +1,52 @@
+from __future__ import annotations
+
+from datetime import datetime, timezone
+from typing import Optional, List
+
+from sqlalchemy import (
+    Integer, String, Text, DateTime, ForeignKey, Index, func, text,
+)
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+
+from shared.db.base import Base
+
+
+def utcnow() -> datetime:
+    return datetime.now(timezone.utc)
+
+
+class MarketPlace(Base):
+    __tablename__ = "market_places"
+
+    id: Mapped[int] = mapped_column(Integer, primary_key=True)
+    container_type: Mapped[str] = mapped_column(
+        String(32), nullable=False, server_default=text("'page'"),
+    )
+    container_id: Mapped[int] = mapped_column(Integer, nullable=False)
+    name: Mapped[str] = mapped_column(String(255), nullable=False)
+    slug: Mapped[str] = mapped_column(String(255), nullable=False)
+    description: 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(),
+    )
+    deleted_at: Mapped[Optional[datetime]] = mapped_column(
+        DateTime(timezone=True), nullable=True,
+    )
+
+    nav_tops: Mapped[List["NavTop"]] = relationship(
+        "NavTop", back_populates="market",
+    )
+
+    __table_args__ = (
+        Index("ix_market_places_container", "container_type", "container_id"),
+        Index(
+            "ux_market_places_slug_active",
+            func.lower(slug),
+            unique=True,
+            postgresql_where=text("deleted_at IS NULL"),
+        ),
+    )