perf(voice): run STT+TTS on the GPU by default with CPU fallback

Both voice backends defaulted to CPU. Fix the "CUDA unavailable" gaps so
everything that benefits from the RTX 5050 uses it:

- MeloTTS venv had CPU-only torch (2.12.0+cpu) -> installed Blackwell-capable
  torch/torchaudio 2.11.0+cu128 (sm_120 verified with a real GPU matmul).
- faster-whisper CUDA loaded but transcribe() died with "libcublas.so.12 not
  found": installed nvidia-cublas-cu12 + nvidia-cudnn-cu12 into the whisper
  venv and inject those nvidia/*/lib dirs into the worker's LD_LIBRARY_PATH at
  spawn (the loader only honours it at exec).
- WSAI_WHISPER_DEVICE / WSAI_MELO_DEVICE now default to "auto": pick CUDA when
  present, else CPU, and each worker falls back to CPU if a CUDA load fails so
  the voice loop never dies on a GPU-less host.

Verified end-to-end through the real backend classes: both workers report
"ready on cuda"; steady-state STT ~170ms (was ~1350ms CPU), TTS ~4s first call
vs ~23s CPU. All 12 tests pass.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

wsai/backends/melo.py

@@ -9,7 +9,8 @@ integration later swaps the sink for "play this wav into the call".
 Env:
     WSAI_MELO_PYTHON   interpreter with melo installed
                        (default: /home/claude/jarvis-tts/melo311/bin/python)
-    WSAI_MELO_DEVICE   cpu | cuda | auto  (default cpu; cuda needs GPU approval)
+    WSAI_MELO_DEVICE   cpu | cuda | auto  (default auto: GPU if torch sees one,
+                       else CPU; the worker falls back to CPU if CUDA fails)
     WSAI_TTS_OUT_DIR   where wavs are written (default ~/.cache/wsai/tts)
     WSAI_TTS_SPEED     synthesis speed multiplier (default 1.3)
 """
@@ -50,7 +51,7 @@ class MeloTTS:
         sink: Sink | None = None,
     ) -> None:
         self.python = python or os.environ.get("WSAI_MELO_PYTHON", _DEFAULT_PYTHON)
-        self.device = device or os.environ.get("WSAI_MELO_DEVICE", "cpu")
+        self.device = device or os.environ.get("WSAI_MELO_DEVICE", "auto")
         self.out_dir = Path(out_dir or os.environ.get("WSAI_TTS_OUT_DIR")
                             or (Path.home() / ".cache/wsai/tts"))
         self.speed = float(speed if speed is not None