feat: decouple market from shared_lib, add app-owned models

Phase 1-3 of decoupling:
- path_setup.py adds project root to sys.path
- Market-owned models in market/models/ (market, market_place)
- All imports updated: shared.infrastructure, shared.db, shared.browser, etc.
- MarketPlace uses container_type/container_id instead of post_id FK

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"),
+        ),
+    )