feat: extract shared infrastructure from shared_lib

Phase 1-3 of decoupling plan:
- Shared DB, models, infrastructure, browser, config, utils
- Event infrastructure (domain_events outbox, bus, processor)
- Structured logging
- Generic container concept (container_type/container_id)
- Alembic migrations for all schema changes

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

models/menu_item.py

@@ -0,0 +1,42 @@
+from datetime import datetime
+from typing import Optional
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+from sqlalchemy import Integer, String, DateTime, ForeignKey, func
+from shared.db.base import Base
+
+
+class MenuItem(Base):
+    __tablename__ = "menu_items"
+
+    id: Mapped[int] = mapped_column(Integer, primary_key=True, autoincrement=True)
+
+    # Foreign key to posts table
+    post_id: Mapped[int] = mapped_column(
+        Integer,
+        ForeignKey("posts.id", ondelete="CASCADE"),
+        nullable=False,
+        index=True
+    )
+
+    # Order for sorting menu items
+    sort_order: Mapped[int] = mapped_column(Integer, nullable=False, default=0, index=True)
+
+    # Timestamps
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True),
+        server_default=func.now(),
+        nullable=False
+    )
+    updated_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True),
+        server_default=func.now(),
+        onupdate=func.now(),
+        nullable=False
+    )
+    deleted_at: Mapped[Optional[datetime]] = mapped_column(
+        DateTime(timezone=True),
+        nullable=True
+    )
+
+    # Relationship to Post
+    post: Mapped["Post"] = relationship("Post", back_populates="menu_items")