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/__init__.py

@@ -9,3 +9,19 @@ from .ghost_membership_entities import (
     GhostTier, GhostSubscription,
 )
 from .domain_event import DomainEvent
+
+from .ghost_content import Tag, Post, Author, PostAuthor, PostTag, PostLike
+from .page_config import PageConfig
+from .order import Order, OrderItem
+from .market import (
+    Product, ProductLike, ProductImage, ProductSection,
+    NavTop, NavSub, Listing, ListingItem,
+    LinkError, LinkExternal, SubcategoryRedirect, ProductLog,
+    ProductLabel, ProductSticker, ProductAttribute, ProductNutrition, ProductAllergen,
+    CartItem,
+)
+from .market_place import MarketPlace
+from .calendars import (
+    Calendar, CalendarEntry, CalendarSlot,
+    TicketType, Ticket, CalendarEntryPost,
+)