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/organization.py

@@ -0,0 +1,18 @@
+"""Organization model."""
+from datetime import datetime
+
+from sqlalchemy import Boolean, Column, DateTime, Integer, String
+
+from app.db.session import Base
+
+
+class Organization(Base):
+    """Organization model for grouping users."""
+
+    __tablename__ = "organizations"
+
+    id = Column(Integer, primary_key=True, index=True)
+    name = Column(String, nullable=False, unique=True, index=True)
+    description = Column(String, nullable=True)
+    is_active = Column(Boolean, default=True, nullable=False)
+    created_at = Column(DateTime, nullable=False, default=datetime.utcnow)