Fix database migrations for Railway deployment

- Add InviteCode model import to alembic env.py
- Derive DATABASE_URL_SYNC from DATABASE_URL as a property
  so it uses the same Railway-provided connection string

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

backend/alembic/env.py

@@ -9,7 +9,7 @@ from alembic import context
 
 # Import your models
 from app.core.database import Base
-from app.models import User, Team, Tree, Session, Attachment
+from app.models import User, Team, Tree, Session, Attachment, InviteCode
 from app.core.config import settings
 
 # this is the Alembic Config object