Monorepo: consolidate 7 repos into one

Combines shared, blog, market, cart, events, federation, and account
into a single repository. Eliminates submodule sync, sibling model
copying at build time, and per-app CI orchestration.

Changes:
- Remove per-app .git, .gitmodules, .gitea, submodule shared/ dirs
- Remove stale sibling model copies from each app
- Update all 6 Dockerfiles for monorepo build context (root = .)
- Add build directives to docker-compose.yml
- Add single .gitea/workflows/ci.yml with change detection
- Add .dockerignore for monorepo build context
- Create __init__.py for federation and account (cross-app imports)

shared/models/menu_item.py

@@ -0,0 +1,37 @@
+from datetime import datetime
+from typing import Optional
+from sqlalchemy.orm import Mapped, mapped_column
+from sqlalchemy import Integer, String, DateTime, ForeignKey, func
+from shared.db.base import Base
+
+
+class MenuItem(Base):
+    """Deprecated — kept so the table isn't dropped. Use shared.models.menu_node.MenuNode."""
+    __tablename__ = "menu_items"
+
+    id: Mapped[int] = mapped_column(Integer, primary_key=True, autoincrement=True)
+
+    post_id: Mapped[int] = mapped_column(
+        Integer,
+        ForeignKey("posts.id", ondelete="CASCADE"),
+        nullable=False,
+        index=True
+    )
+
+    sort_order: Mapped[int] = mapped_column(Integer, nullable=False, default=0, index=True)
+
+    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
+    )