Fix sidecar crash: torch circular import under PyInstaller

- Exclude ctranslate2.converters from PyInstaller bundle — these modules
  import torch at module level causing circular import crashes, and are
  only needed for model conversion (never used at runtime)
- Defer all heavy ML imports to first handler call instead of startup,
  so the sidecar can send its ready message without loading torch/whisper

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

python/voice_to_notes.spec

@@ -33,7 +33,13 @@ a = Analysis(
     hookspath=[],
     hooksconfig={},
     runtime_hooks=[],
-    excludes=["tkinter", "test", "unittest", "pip", "setuptools"],
+    excludes=[
+        "tkinter", "test", "unittest", "pip", "setuptools",
+        # ctranslate2.converters imports torch at module level and causes
+        # circular import crashes under PyInstaller. These modules are only
+        # needed for model format conversion, never for inference.
+        "ctranslate2.converters",
+    ],
     win_no_prefer_redirects=False,
     win_private_assemblies=False,
     cipher=block_cipher,