Decouple cart: use shared.models for all cross-app imports

- Replace all imports from blog.models, market.models, events.models
  and bare models.* with shared.models equivalents
- Convert cart/models/order.py and page_config.py to re-export stubs
- Update shared + glue submodule pointers

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

models/page_config.py

@@ -1,39 +1 @@
-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
-    )
+from shared.models.page_config import PageConfig  # noqa: F401