Java Spring-Boot & Core-Java senior 4 matrix.org API implementation

We look for Java Spring-Boog and Core-Java senior expert developer

Your job will be to implement requirements which are mutually agreed on this API
https://playground.matrix.org/#overview

maybe helpful:
https://matrix.org/docs/matrix-concepts/elements-of-matrix/
https://matrix.org/docs/chat_basics/matrix-for-im/

Mandatory!!!!
You have to have a installed matrix synapse on PC/Laptop and on your smartphone to run on your local environment. You document EACH step

you can run locally via:
https://hub.docker.com/r/matrixdotorg/synapse

Milestones
MS1:
Implement a spring boot service to invite users to the matrix.
- sendNewUserRegistrations(List<UserDTO>)
send a template e-mail to users to invite them to register an account
- createNewPrivateGroupChats(List<GroupChatDto>, SpaceDto)
create the list of (private) group chats into the given Space
- inviteNewUsers(List<UserDTO>, List<GroupChatDto>)
invite the given list of users to the list of GroupChats
- elevatePermissions(List<UserDTO>, List<RoleDTO>, List<GroupChatDto>)
elevate the given users into the given Roles (e.g. Moderator, Admin) on the given Chats
- remoteAllPermissions(List<UserDTO>, List<GroupChatDto>)
remove the given user's permissions on the given Chats
- remoteUsersFromGroups(List<UserDTO>, List<GroupChatDto>)
remove the given user's completely from the given chats

MS2:
Implement a spring boot service to send matrix messages on
- direct messages
sendMatrixDirectMessages(List<MessageDto>, List<UserDto>)
- group messages
sendMatrixGroupMessages(List<MessageDto>, List<GroupChatDto>)

MS-later:
- implement a bridge to work via e-mail between group chat and e-mail mailbox

Implementations:
- a Interface representing the above methods
- the impl of the interface
- the DTOs are hardcoded examples in multiple JUnit tests, so that the JUnit tests is basically your "main" method replacement with multiple entry points

NO UI for now required!
NO database required!
No REST endpoint exposing for the above named methods required! (only consuming the apis in the given links)

Your background is:
- multiple years of experience with Java
- multiple years of experience with REST APIs

If you are a good fit, you are open to get more tasks about implementing solutions fully on your own (e.g. with your team)

Budget?
will not be disclosed, place your best bid to get considered

What is next?
We will share you a NDA and afterwards a paid test task.

Payment?
- you estimate in a WBS (optimistic, expected, pessimistic, where optimistic < expected < pessimistic) after getting the task
- we discuss about clearances and effort
- we mutually agree to effort
- we assign you the task after mutually agreed
- you implement, test and deliver
- you provide also unit /integration tests to ensure code is always working
- you document each required step to make interaction of systems to work
- we test on our premise (you maybe help to fix issues)
- we pay
(basically the rules of freelancer)

Closed book vs open book?
We work only on open book.
Closed book means you are unwilling to define a WBS for the work and you add only a price tag to the task.
We are sorry we will not hire you in such a case!

Deliveries?
- in our on premise git (access will be granted to you)
- full sources
- mutliple samples of body and responses of the api in test/resources which will be used as unit tests
- maven
- libs, need prior confirm and we prefer to use mostly latest stable versions
- JDK 17 (mostly LTS)
- your api access code working on our end
- provided or fully documented steps to start the docker container the full runtime