add: absorb soprano_to_rvc as regular subdirectory

Voice conversion pipeline (Soprano TTS → RVC) with Docker support.
Previously tracked as bare gitlink; removed .git/ directories and
absorbed into main repo for unified tracking.

Includes: Soprano TTS, RVC WebUI integration, Docker configs,
WebSocket API, and benchmark scripts.
Updated .gitignore to exclude large model weights (*.pth, *.pt, *.onnx, *.index).
287 files (3.1GB of ML weights properly excluded via gitignore).

soprano_to_rvc/soprano/.gitignore

@@ -0,0 +1,11 @@
+__pycache__/
+test.py
+*.wav
+dist/
+*.egg-info/
+.venv/
+.task/
+BACKEND-SELECTION.md
+Taskfile.yml
+benchmark.py
+examples-backend.py