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/tts.py

@@ -0,0 +1,34 @@
+"""
+TTS inference module - delegates to backend abstraction layer.
+"""
+
+from typing import Optional
+import numpy as np
+import io
+import soundfile as sf
+
+from ..backends import get_tts_backend, TTSBackend
+
+
+def get_tts_model() -> TTSBackend:
+    """
+    Get TTS backend instance (MLX or PyTorch based on platform).
+    
+    Returns:
+        TTS backend instance
+    """
+    return get_tts_backend()
+
+
+def unload_tts_model():
+    """Unload TTS model to free memory."""
+    backend = get_tts_backend()
+    backend.unload_model()
+
+
+def audio_to_wav_bytes(audio: np.ndarray, sample_rate: int) -> bytes:
+    """Convert audio array to WAV bytes."""
+    buffer = io.BytesIO()
+    sf.write(buffer, audio, sample_rate, format="WAV")
+    buffer.seek(0)
+    return buffer.read()