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/health.py

@@ -8,9 +8,10 @@ import torch
 from fastapi import APIRouter, Depends
 from sqlalchemy.orm import Session
 
-from .. import config, models, tts
+from .. import config, models
+from ..services import tts
 from ..database import get_db
-from ..platform_detect import get_backend_type
+from ..utils.platform_detect import get_backend_type
 
 router = APIRouter()