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/history.py

@@ -7,7 +7,8 @@ from fastapi import APIRouter, Depends, File, HTTPException, UploadFile
 from fastapi.responses import FileResponse, StreamingResponse
 from sqlalchemy.orm import Session
 
-from .. import export_import, history, models
+from .. import models
+from ..services import export_import, history
 from ..app import safe_content_disposition
 from ..database import Generation as DBGeneration, VoiceProfile as DBVoiceProfile, get_db