docs: add docs to CB

Pretty much every class and method should have documentation now.

ref: N25B-295

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

@@ -6,4 +6,7 @@ router = APIRouter()
 # TODO: implement
 @router.get("/sse")
 async def sse(request: Request):
+    """
+    Placeholder for future Server-Sent Events endpoint.
+    """
     pass