feat: add PageConfig model and remove duplicate CartItem

Introduce page_configs table for per-page feature flags (calendar, market)
and future SumUp credentials. Add page_config relationship to Post model.
Remove duplicate CartItem definition from cart_item.py (canonical stays in market.py).

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

models/page_config.py

@@ -0,0 +1,45 @@
+from __future__ import annotations
+
+from datetime import datetime
+from typing import Optional
+
+from sqlalchemy import Integer, String, Text, DateTime, ForeignKey, func, JSON
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+
+from db.base import Base
+
+
+class PageConfig(Base):
+    __tablename__ = "page_configs"
+
+    id: Mapped[int] = mapped_column(Integer, primary_key=True, autoincrement=True)
+
+    post_id: Mapped[int] = mapped_column(
+        Integer,
+        ForeignKey("posts.id", ondelete="CASCADE"),
+        unique=True,
+        nullable=False,
+    )
+
+    features: Mapped[dict] = mapped_column(
+        JSON, nullable=False, server_default="{}"
+    )
+
+    # Phase 3: 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
+    )
+
+    post: Mapped["Post"] = relationship(
+        "Post", back_populates="page_config", foreign_keys=[post_id]
+    )