Assign Member Tier

Assigns a member tier to a member. Example curl: ```bash curl --request POST '{{base_url}}/sp/member-tiers/assign' \ --header 'Authorization: Bearer {{access_token}}' \ --header 'Content-Type: application/json' \ --data-raw '{ "idempotency_key": "{{$timestamp}}-tier-assign", "contact_id": {{contact_id}}, "tier_id": {{tier_id}}, "start_date": "2026-04-06", "expiry_date": "2027-04-06" }' ``` Example success response shape: ```json { "success": true, "message": "Member tier assigned successfully.", "data": { "contact_id": 456, "tier_id": 1, "assigned": true, "expiry_date": "2027-04-06" }, "meta": {} } ``` Example error response shape: ```json { "success": false, "message": "The same idempotency_key cannot be reused with a different payload.", "error": { "code": "idempotency_key_reused_with_different_payload", "details": {} } } ``` Common platform errors: - `idempotency_key_required` - `member_not_found` - `sp_scope_not_granted` - `sp_token_scope_missing` - `legacy_field_not_allowed` - `query_parameters_not_allowed` - `invalid_content_type`