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/booking.py

@@ -1,7 +1,7 @@
 """Booking model."""
 from datetime import datetime
 
-from sqlalchemy import Column, DateTime, ForeignKey, Integer, String
+from sqlalchemy import Boolean, Column, DateTime, ForeignKey, Integer, String
 from sqlalchemy.orm import relationship
 
 from app.db.session import Base
@@ -13,8 +13,12 @@ class Booking(Base):
     __tablename__ = "bookings"
 
     id = Column(Integer, primary_key=True, index=True)
-    user_id = Column(Integer, ForeignKey("users.id"), nullable=False, index=True)
+    user_id = Column(Integer, ForeignKey("users.id"), nullable=True, index=True)
     space_id = Column(Integer, ForeignKey("spaces.id"), nullable=False, index=True)
+    guest_name = Column(String, nullable=True)
+    guest_email = Column(String, nullable=True)
+    guest_organization = Column(String, nullable=True)
+    is_anonymous = Column(Boolean, default=False, nullable=False)
     title = Column(String, nullable=False)
     description = Column(String, nullable=True)
     start_datetime = Column(DateTime, nullable=False, index=True)