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/page_config.py

@@ -0,0 +1,39 @@
+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
+    )