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_member.py

@@ -0,0 +1,17 @@
+"""OrganizationMember junction model."""
+from sqlalchemy import Column, ForeignKey, Integer, String, UniqueConstraint
+
+from app.db.session import Base
+
+
+class OrganizationMember(Base):
+    """Junction table linking organizations to their members."""
+
+    __tablename__ = "organization_members"
+
+    id = Column(Integer, primary_key=True, index=True)
+    organization_id = Column(Integer, ForeignKey("organizations.id"), nullable=False, index=True)
+    user_id = Column(Integer, ForeignKey("users.id"), nullable=False, index=True)
+    role = Column(String, nullable=False, default="member")  # "admin" or "member"
+
+    __table_args__ = (UniqueConstraint("organization_id", "user_id", name="uq_org_member"),)