feat: add multi-tenant system with properties, organizations, and public booking

Implement complete multi-property architecture:
- Properties (groups of spaces) with public/private visibility
- Property managers (many-to-many) with role-based permissions
- Organizations with member management
- Anonymous/guest booking support via public API (/api/public/*)
- Property-scoped spaces, bookings, and settings
- Frontend: property selector, organization management, public booking views
- Migration script and updated seed data

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/models/__init__.py

@@ -5,8 +5,19 @@ from app.models.booking import Booking
 from app.models.booking_template import BookingTemplate
 from app.models.google_calendar_token import GoogleCalendarToken
 from app.models.notification import Notification
+from app.models.organization import Organization
+from app.models.organization_member import OrganizationMember
+from app.models.property import Property
+from app.models.property_access import PropertyAccess
+from app.models.property_manager import PropertyManager
+from app.models.property_settings import PropertySettings
 from app.models.settings import Settings
 from app.models.space import Space
 from app.models.user import User
 
-__all__ = ["User", "Space", "Settings", "Booking", "BookingTemplate", "Notification", "AuditLog", "Attachment", "GoogleCalendarToken"]
+__all__ = [
+    "User", "Space", "Settings", "Booking", "BookingTemplate",
+    "Notification", "AuditLog", "Attachment", "GoogleCalendarToken",
+    "Property", "PropertyManager", "PropertyAccess", "PropertySettings",
+    "Organization", "OrganizationMember",
+]