CRITICAL FIX: Add missing demo_account_type to gateway middleware

The gateway middleware was not including demo_account_type in the
request.state.user context, causing the tenant API to filter with
an empty account type.

## The Bug:

Gateway middleware set:
- demo_session_id ✅
- is_demo ✅
- demo_account_type ❌ MISSING!

This caused get_virtual_tenants_for_session() to be called with
demo_account_type="" (empty string), which returned only the parent
tenant instead of parent + 5 children.

## Log Evidence:

Before fix:
  Demo session detected for get_user_tenants
    demo_account_type=   ← EMPTY!
    tenant_count=1       ← Only parent!

After fix (expected):
  Demo session detected for get_user_tenants
    demo_account_type=enterprise
    tenant_count=6       ← Parent + 5 children!

## Fix:

Added line 211 in gateway/app/middleware/demo_middleware.py:
  "demo_account_type": session_info.get("demo_account_type", "professional")

This ensures the tenant service knows whether it's an enterprise or
professional demo session and returns the correct tenant list.

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

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

gateway/app/middleware/demo_middleware.py

@@ -208,6 +208,7 @@ class DemoMiddleware(BaseHTTPMiddleware):
                         "role": "owner",  # Demo users have owner role
                         "is_demo": True,
                         "demo_session_id": session_id,
+                        "demo_account_type": session_info.get("demo_account_type", "professional"),
                         "demo_session_status": current_status
                     }