- Added `using EnvelopeGenerator.API.Extensions;` for utilities.
- Changed `SignatureCacheKeyPrefix` to a new prefix value.
- Added `[Authorize(Policy = AuthPolicy.Receiver)]` to methods.
- Used `[FromRoute]` for `envelopeKey` in route-bound methods.
- Updated cache key logic to use `User.ReceiverSignature`.
- Made `DataUrl`, `FullName`, and `Place` required in `SignatureCacheRequest`.
- Set default value (`null`) for `Position` in `SignatureCacheRequest`.