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/ipc/handlers.py

@@ -41,11 +41,15 @@ def ping_handler(msg: IPCMessage) -> IPCMessage:
 
 def make_transcribe_handler() -> HandlerFunc:
     """Create a transcription handler with a persistent TranscribeService."""
-    from voice_to_notes.services.transcribe import TranscribeService, result_to_payload
-
-    service = TranscribeService()
+    service = None
 
     def handler(msg: IPCMessage) -> IPCMessage:
+        nonlocal service
+        if service is None:
+            from voice_to_notes.services.transcribe import TranscribeService
+            service = TranscribeService()
+        from voice_to_notes.services.transcribe import result_to_payload
+
         payload = msg.payload
         result = service.transcribe(
             request_id=msg.id,
@@ -66,11 +70,15 @@ def make_transcribe_handler() -> HandlerFunc:
 
 def make_diarize_handler() -> HandlerFunc:
     """Create a diarization handler with a persistent DiarizeService."""
-    from voice_to_notes.services.diarize import DiarizeService, diarization_to_payload
-
-    service = DiarizeService()
+    service = None
 
     def handler(msg: IPCMessage) -> IPCMessage:
+        nonlocal service
+        if service is None:
+            from voice_to_notes.services.diarize import DiarizeService
+            service = DiarizeService()
+        from voice_to_notes.services.diarize import diarization_to_payload
+
         payload = msg.payload
         result = service.diarize(
             request_id=msg.id,
@@ -163,11 +171,15 @@ def make_diarize_download_handler() -> HandlerFunc:
 
 def make_pipeline_handler() -> HandlerFunc:
     """Create a full pipeline handler (transcribe + diarize + merge)."""
-    from voice_to_notes.services.pipeline import PipelineService, pipeline_result_to_payload
-
-    service = PipelineService()
+    service = None
 
     def handler(msg: IPCMessage) -> IPCMessage:
+        nonlocal service
+        if service is None:
+            from voice_to_notes.services.pipeline import PipelineService
+            service = PipelineService()
+        from voice_to_notes.services.pipeline import pipeline_result_to_payload
+
         payload = msg.payload
         result = service.run(
             request_id=msg.id,
@@ -193,11 +205,15 @@ def make_pipeline_handler() -> HandlerFunc:
 
 def make_export_handler() -> HandlerFunc:
     """Create an export handler."""
-    from voice_to_notes.services.export import ExportService, make_export_request
-
-    service = ExportService()
+    service = None
 
     def handler(msg: IPCMessage) -> IPCMessage:
+        nonlocal service
+        if service is None:
+            from voice_to_notes.services.export import ExportService
+            service = ExportService()
+        from voice_to_notes.services.export import make_export_request
+
         request = make_export_request(msg.payload)
         output_path = service.export(request)
         return IPCMessage(
@@ -211,11 +227,14 @@ def make_export_handler() -> HandlerFunc:
 
 def make_ai_chat_handler() -> HandlerFunc:
     """Create an AI chat handler with persistent AIProviderService."""
-    from voice_to_notes.services.ai_provider import create_default_service
-
-    service = create_default_service()
+    service = None
 
     def handler(msg: IPCMessage) -> IPCMessage:
+        nonlocal service
+        if service is None:
+            from voice_to_notes.services.ai_provider import create_default_service
+            service = create_default_service()
+
         payload = msg.payload
         action = payload.get("action", "chat")