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/audio.py

@@ -6,7 +6,8 @@ from fastapi import APIRouter, Depends, HTTPException
 from fastapi.responses import FileResponse
 from sqlalchemy.orm import Session
 
-from .. import history, models
+from .. import models
+from ..services import history
 from ..database import get_db
 
 router = APIRouter()
@@ -15,7 +16,7 @@ router = APIRouter()
 @router.get("/audio/version/{version_id}")
 async def get_version_audio(version_id: str, db: Session = Depends(get_db)):
     """Serve audio for a specific version."""
-    from .. import versions as versions_mod
+    from ..services import versions as versions_mod
 
     version = versions_mod.get_version(version_id, db)
     if not version: