chore: filled in project structure

Added some example basic files containing a functioning /message
endpoint which logs the received message to INFO.

ref: N25B-144

src/control_backend/api/v1/endpoints/sse.py

@@ -0,0 +1,8 @@
+from fastapi import APIRouter, Request
+
+router = APIRouter()
+
+# TODO: implement
+@router.get("/sse")
+async def sse(request: Request):
+    pass