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/data/demo_data.json

@@ -368,21 +368,20 @@
             "name": "E-Commerce Platform Redesign",
             "slug": "ecommerce-redesign",
             "description": "Complete redesign of the e-commerce platform with modern UX, improved checkout flow, and mobile-first approach.",
-            "owner_email": "demo@example.com",
+            "owner_email": "__admin__",
             "autonomy_level": "milestone",
             "status": "active",
             "complexity": "complex",
             "client_mode": "technical",
             "settings": {
-                "mcp_servers": ["gitea", "knowledge-base"],
-                "notification_email": "demo@example.com"
+                "mcp_servers": ["gitea", "knowledge-base"]
             }
         },
         {
             "name": "Mobile Banking App",
             "slug": "mobile-banking",
             "description": "Secure mobile banking application with biometric authentication, transaction history, and real-time notifications.",
-            "owner_email": "alice@acme.com",
+            "owner_email": "__admin__",
             "autonomy_level": "full_control",
             "status": "active",
             "complexity": "complex",
@@ -396,7 +395,7 @@
             "name": "Internal HR Portal",
             "slug": "hr-portal",
             "description": "Employee self-service portal for leave requests, performance reviews, and document management.",
-            "owner_email": "carol@globex.com",
+            "owner_email": "__admin__",
             "autonomy_level": "autonomous",
             "status": "active",
             "complexity": "medium",