move CRUD and service modules into services/, platform_detect into utils/

Move 9 business-logic modules from the backend root into services/:
channels, effects, history, profiles, stories, versions, export_import,
transcribe, tts. Move platform_detect.py into utils/.

Backend root now contains only infrastructure (app, main, config, server,
models, build_binary) and docs. All 94 routes verified.

backend/routes/transcription.py

@@ -6,7 +6,8 @@ from pathlib import Path
 
 from fastapi import APIRouter, File, Form, HTTPException, UploadFile
 
-from .. import models, transcribe
+from .. import models
+from ..services import transcribe
 from ..services.task_queue import create_background_task
 from ..utils.tasks import get_task_manager