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/space.py

@@ -1,5 +1,6 @@
 """Space model."""
-from sqlalchemy import Boolean, Column, Integer, String
+from sqlalchemy import Boolean, Column, ForeignKey, Integer, String
+from sqlalchemy.orm import relationship
 
 from app.db.session import Base
 
@@ -15,9 +16,12 @@ class Space(Base):
     capacity = Column(Integer, nullable=False)
     description = Column(String, nullable=True)
     is_active = Column(Boolean, default=True, nullable=False)
+    property_id = Column(Integer, ForeignKey("properties.id"), nullable=True, index=True)
 
     # Per-space scheduling settings (NULL = use global default)
     working_hours_start = Column(Integer, nullable=True)
     working_hours_end = Column(Integer, nullable=True)
     min_duration_minutes = Column(Integer, nullable=True)
     max_duration_minutes = Column(Integer, nullable=True)
+
+    property = relationship("Property", backref="spaces")