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/user.py

@@ -14,7 +14,7 @@ class User(Base):
     email = Column(String, unique=True, index=True, nullable=False)
     full_name = Column(String, nullable=False)
     hashed_password = Column(String, nullable=False)
-    role = Column(String, nullable=False, default="user")  # "admin" or "user"
+    role = Column(String, nullable=False, default="user")  # "superadmin"/"manager"/"user"
     organization = Column(String, nullable=True)
     is_active = Column(Boolean, default=True, nullable=False)
     timezone = Column(String(50), default="UTC", nullable=False)  # IANA timezone
@@ -26,3 +26,5 @@ class User(Base):
     google_calendar_token = relationship(
         "GoogleCalendarToken", back_populates="user", uselist=False
     )
+    managed_properties = relationship("PropertyManager", backref="user", cascade="all, delete-orphan")
+    organization_memberships = relationship("OrganizationMember", backref="user", cascade="all, delete-orphan")