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/v1/endpoints/client_profile.py

@@ -50,11 +50,49 @@ async def get_current_client_profile(
     profile = result.scalar_one_or_none()
     
     if not profile:
-        # Si no existe, crear uno vacío
-        profile = ClientProfile(tenant_id=current_tenant.id)
-        db.add(profile)
-        await db.commit()
-        await db.refresh(profile)
+        # Si no existe, devolver un perfil vacío con solo tenant_id
+        # No crear en base de datos hasta que el usuario guarde
+        return ClientProfileResponse(
+            id=None,
+            tenant_id=current_tenant.id,
+            business_name=None,
+            commercial_name=None,
+            client_code=None,
+            client_type=None,
+            rfc=None,
+            tax_id=None,
+            country=None,
+            state=None,
+            city=None,
+            address=None,
+            external_number=None,
+            internal_number=None,
+            postal_code=None,
+            neighborhood=None,
+            main_phone=None,
+            secondary_phone=None,
+            direct_phone=None,
+            phone_extension=None,
+            fax=None,
+            business_hours=None,
+            website=None,
+            main_email=None,
+            billing_email=None,
+            advertising_medium=None,
+            nationality=None,
+            logo_url=None,
+            company_representative=None,
+            legal_representative=None,
+            credit_limit=None,
+            payment_terms=None,
+            preferred_currency="MXN",
+            send_to_billing=False,
+            is_active_client=True,
+            is_prospect=False,
+            notes=None,
+            created_at=None,
+            updated_at=None
+        )
     
     return profile