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/schemas/organization.py

@@ -0,0 +1,41 @@
+"""Organization schemas."""
+from datetime import datetime
+
+from pydantic import BaseModel, Field
+
+
+class OrganizationCreate(BaseModel):
+    name: str = Field(..., min_length=1)
+    description: str | None = None
+
+
+class OrganizationUpdate(BaseModel):
+    name: str | None = None
+    description: str | None = None
+
+
+class OrganizationResponse(BaseModel):
+    id: int
+    name: str
+    description: str | None = None
+    is_active: bool
+    created_at: datetime
+    member_count: int = 0
+
+    model_config = {"from_attributes": True}
+
+
+class OrganizationMemberResponse(BaseModel):
+    id: int
+    organization_id: int
+    user_id: int
+    role: str
+    user_name: str | None = None
+    user_email: str | None = None
+
+    model_config = {"from_attributes": True}
+
+
+class AddMemberRequest(BaseModel):
+    user_id: int
+    role: str = "member"