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/services/transcribe.py

@@ -0,0 +1,22 @@
+"""
+STT (Speech-to-Text) module - delegates to backend abstraction layer.
+"""
+
+from typing import Optional
+from ..backends import get_stt_backend, STTBackend
+
+
+def get_whisper_model() -> STTBackend:
+    """
+    Get STT backend instance (MLX or PyTorch based on platform).
+    
+    Returns:
+        STT backend instance
+    """
+    return get_stt_backend()
+
+
+def unload_whisper_model():
+    """Unload Whisper model to free memory."""
+    backend = get_stt_backend()
+    backend.unload_model()