feat(demo): tie all demo projects to admin user

- Update demo_data.json to use "__admin__" as owner_email for all projects
- Add admin user lookup in load_demo_data() with special "__admin__" key
- Remove notification_email from project settings (not a valid field)

This ensures demo projects are visible to the admin user when logged in.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/app/init_db.py

@@ -267,6 +267,15 @@ async def load_demo_data(session: AsyncSession) -> None:
 
         await session.flush()
 
+        # Add admin user to map with special "__admin__" key
+        # This allows demo data to reference the admin user as owner
+        superuser_email = settings.FIRST_SUPERUSER_EMAIL or "admin@example.com"
+        admin_user = await user_crud.get_by_email(session, email=superuser_email)
+        if admin_user:
+            user_map["__admin__"] = admin_user
+            user_map[str(admin_user.email)] = admin_user
+            logger.debug(f"Added admin user to map: {admin_user.email}")
+
         # ========================
         # 3. Load Agent Types Map (for FK resolution)
         # ========================