feat(backend): Fix client-profile endpoint and add attachment support

- Fixed client-profile GET endpoint to prevent 500 errors
- Made ClientProfileResponse fields optional (id, created_at, updated_at)
- Returns empty profile data instead of creating DB entry on GET
- Added new attachment model and schemas for file handling
- Added file handler core utility for upload management

backend/app/api/schemas/client_profile.py

@@ -133,10 +133,10 @@ class ClientProfileUpdate(ClientProfileBase):
 class ClientProfileResponse(ClientProfileBase):
     """Schema de respuesta para ClientProfile."""
     
-    id: uuid.UUID
+    id: Optional[uuid.UUID] = None
     tenant_id: uuid.UUID
-    created_at: datetime
-    updated_at: datetime
+    created_at: Optional[datetime] = None
+    updated_at: Optional[datetime] = None
 
     class Config:
         from_attributes = True