Adds UploadPhotoCommand/Handler/Validator and PhotoUploadRepository, invoked via MediatR by other features' commands (e.g. a future CreateBreweryPhoto) rather than exposed through its own controller. Wires the feature and IFileStorageProvider into API.Core.
Introduces IFileStorageProvider and an S3-compatible implementation (S3FileStorageProvider) configured via new SEAWEEDFS_* configuration keys, for storing uploaded files against SeaweedFS.
