Refactor controllers to reduce business-logic coupling (starting with create-room)

- Context
Certain backend controllers nowadays include extensive business logic.
which is tightly integrated with HTTP processing in a domain and makes testing
and, hence, management.

- exemple concret
In `functions/create-room/src/main.js`, the
- Creates room metadata based on input parameters
- Calls LiveKit to create a room
- Stores room data with Appwrite
There is a blending of handling requests and orchestrating business domains.

- Proposal
As a first, well-scoped step:
- Abstract room creation logic into a service/use case. Keep the controller concerned with request parsing and response handling. This will be a non-breaking refactor**, starting with one controller. I will proceed further with examples only if I receive feedback on what I