fix(import): decouple scan from HTTP connection, prevent duplicate imports

- Add scan_manager: background asyncio task + Redis event store so scans
  survive UI navigation; SSE stream reads from Redis and is reconnectable
- Replace SSE-tied scan endpoint with POST /nc-scan/start + GET /nc-scan/stream
- Fix frontend: AbortController + useEffect cleanup cancels stream on unmount
  without stopping the server-side scan
- Add unique constraint on audio_versions.nc_file_path (migration 0009) to
  prevent duplicate imports from concurrent scans; handle IntegrityError
  gracefully in nc_scan with rollback + skip
- Fix API health check: use plain python instead of uv (not in dev image)
- Optimize Taskfile: fix duplicate dev:restart, add dev:fresh/dev:rebuild/
  dev:status, migrate uses run --rm, check includes typecheck

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

docker-compose.yml

@@ -60,7 +60,7 @@ services:
       redis:
         condition: service_healthy
     healthcheck:
-      test: ["CMD-SHELL", "uv run python -c \"import httpx; exit(0 if httpx.get('http://localhost:8000/api/health').status_code == 200 else 1)\" || exit 1"]
+      test: ["CMD-SHELL", "python -c \"import httpx; exit(0 if httpx.get('http://localhost:8000/api/health').status_code == 200 else 1)\" || exit 1"]
       interval: 20s
       timeout: 10s
       retries: 5