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_rvc_config.json

@@ -0,0 +1,18 @@
+{
+  "pth_path": "Retrieval-based-Voice-Conversion-WebUI/assets/weights/MikuAI_e210_s6300.pth",
+  "index_path": "Retrieval-based-Voice-Conversion-WebUI/assets/indices/added_IVF1811_Flat_nprobe_1_MikuAI_v2.index",
+  "pitch": 5,
+  "formant": 0.0,
+  "index_rate": 0.3,
+  "rms_mix_rate": 0.25,
+  "block_time": 0.20,
+  "crossfade_time": 0.05,
+  "extra_time": 1.8,
+  "n_cpu": 4,
+  "f0method": "rmvpe",
+  "soprano_sample_rate": 32000,
+  "rvc_sample_rate": 48000,
+  "use_pv": false,
+  "I_noise_reduce": false,
+  "O_noise_reduce": false
+}