fix(ai): gate AI tools on per-framework GPU detection, not a shared boolean (#445)

gpu_available() answers "can ANY framework use a GPU" (torch, then ONNX, then
paddle). But torch tools consumed that shared boolean directly as
device = torch.device("cuda" if gpu_available() else "cpu"). On a GPU host where
gpu_available() is True via paddle or ONNX while torch is a CPU-only build, those
tools would route to a CUDA torch cannot use and crash. Transcription had the
mirror problem: it runs on CTranslate2 (not torch), so on a transcription-only
GPU box gpu_available() returned False and Whisper ran on CPU despite a GPU.

Add per-framework helpers to gpu.py:
- torch_gpu_available(): torch.cuda.is_available(), honoring SNAPOTTER_GPU.
- ctranslate2_gpu_available(): ctranslate2.get_cuda_device_count() > 0.

Point each tool at the helper for its own framework: upscale, noise_removal,
enhance_faces and restore use torch_gpu_available(); transcribe uses
ctranslate2_gpu_available(). ocr.py keeps gpu_available() (paddle-aware) and the
dispatcher keeps it for its startup GPU-status line. The SNAPOTTER_GPU override
check is factored into a shared _override_disables_gpu() helper.

TDD: 7 new tests in tests/test_gpu_detection.py cover both helpers (override,
CPU-only, absent framework), including the crux that torch_gpu_available() stays
False on a CPU-only torch build even when a GPU exists for another framework.

Claude-Session: https://claude.ai/code/session_01NfaRxjek8ex5nawvx3mVMf

packages/ai/python/tests/test_gpu_detection.py

@@ -11,6 +11,7 @@ GPU-less host and would wedge the shared AI dispatcher).
 import os
 import subprocess
 import sys
+import types
 
 sys.path.insert(0, os.path.join(os.path.dirname(__file__), ".."))
 import gpu  # noqa: E402
@@ -80,3 +81,59 @@ def test_gpu_available_never_probes_paddle_without_a_gpu(monkeypatch):
     monkeypatch.setattr(gpu, "_try_paddle_cuda_subprocess", spy)
     assert gpu.gpu_available() is False
     assert called["paddle"] is False
+
+
+# --- Per-framework detection (torch, ctranslate2) --------------------------
+#
+# Torch and CTranslate2 tools must gate on their OWN framework, not the general
+# gpu_available(), which can report True based on paddle or ONNX Runtime while
+# torch is a CPU-only build. Consuming the shared boolean would make those tools
+# route to CUDA on a device their framework cannot use.
+
+def test_torch_gpu_available_true_when_torch_can_use_cuda(monkeypatch):
+    monkeypatch.delenv("SNAPOTTER_GPU", raising=False)
+    monkeypatch.setattr(gpu, "_try_torch_cuda", lambda: True)
+    assert gpu.torch_gpu_available() is True
+
+
+def test_torch_gpu_available_false_when_override_disables_gpu(monkeypatch):
+    monkeypatch.setenv("SNAPOTTER_GPU", "0")
+    monkeypatch.setattr(gpu, "_try_torch_cuda", lambda: True)
+    assert gpu.torch_gpu_available() is False
+
+
+def test_torch_gpu_available_false_when_torch_is_cpu_only(monkeypatch):
+    # The crux: gpu_available() may be True via paddle or ONNX on a GPU box, but a
+    # CPU-only torch build must report no GPU so torch tools do not touch CUDA.
+    monkeypatch.delenv("SNAPOTTER_GPU", raising=False)
+    monkeypatch.setattr(gpu, "_try_torch_cuda", lambda: False)
+    assert gpu.torch_gpu_available() is False
+
+
+def test_ctranslate2_gpu_available_true_when_cuda_device_present(monkeypatch):
+    monkeypatch.delenv("SNAPOTTER_GPU", raising=False)
+    fake = types.SimpleNamespace(get_cuda_device_count=lambda: 1)
+    monkeypatch.setitem(sys.modules, "ctranslate2", fake)
+    assert gpu.ctranslate2_gpu_available() is True
+
+
+def test_ctranslate2_gpu_available_false_when_no_cuda_device(monkeypatch):
+    monkeypatch.delenv("SNAPOTTER_GPU", raising=False)
+    fake = types.SimpleNamespace(get_cuda_device_count=lambda: 0)
+    monkeypatch.setitem(sys.modules, "ctranslate2", fake)
+    assert gpu.ctranslate2_gpu_available() is False
+
+
+def test_ctranslate2_gpu_available_false_when_override_disables_gpu(monkeypatch):
+    monkeypatch.setenv("SNAPOTTER_GPU", "0")
+    fake = types.SimpleNamespace(get_cuda_device_count=lambda: 4)
+    monkeypatch.setitem(sys.modules, "ctranslate2", fake)
+    assert gpu.ctranslate2_gpu_available() is False
+
+
+def test_ctranslate2_gpu_available_false_when_not_installed(monkeypatch):
+    # A None entry in sys.modules makes `import ctranslate2` raise ImportError,
+    # which models the framework being absent (e.g. no transcription bundle).
+    monkeypatch.delenv("SNAPOTTER_GPU", raising=False)
+    monkeypatch.setitem(sys.modules, "ctranslate2", None)
+    assert gpu.ctranslate2_gpu_available() is False