fix(backends): explain why an explicitly requested device cannot be used

--device nvidia on a machine without one reported "no usable backend remains",
which says nothing about the cause and repeated the same message once per URL.
Asking for a device that cannot run is a configuration mistake, so it now fails
once, fatally, naming the reason: "nvidia/cuda: no nvidia hardware detected".

Found by end-to-end testing; the unit tests only covered the runtime-failure
path, where TranscribeError remains correct.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.gitignore

@@ -17,3 +17,7 @@ out/
 .coverage
 htmlcov/
 .pytest_cache/
+
+# Tool caches
+.mypy_cache/
+.ruff_cache/

src/ccn_transcribe/backends/registry.py

@@ -55,6 +55,20 @@ def _select_requested(
     return matches
 
 
+def explain_unavailable(requested: str, chain: tuple[type[Backend], ...] | None = None) -> str:
+    """Why an explicitly requested backend cannot be used."""
+    chain = chain if chain is not None else DEFAULT_CHAIN
+    reasons: list[str] = []
+    for backend in _select_requested(requested, chain):
+        label = key(backend)
+        if not backend.probe_hardware():
+            reasons.append(f"{label}: no {backend.name} hardware detected")
+            continue
+        status = backend.probe_toolchain()
+        reasons.append(f"{label}: {status.reason or 'unavailable'}")
+    return "; ".join(reasons)
+
+
 def candidates(
     requested: str | None = None, chain: tuple[type[Backend], ...] | None = None
 ) -> Iterator[type[Backend]]:
@@ -149,6 +163,13 @@ def transcribe_with_fallback(
             log.warning("backend %s failed (%s); demoting it and falling back", label, exc)
             last = exc
 
+    if explicit and requested is not None and last is None:
+        # Asking for a backend that cannot run is a configuration mistake, not a
+        # per-job failure; say why once and stop rather than repeating it.
+        raise errors.ConfigError(
+            f"--device {requested} was requested but cannot be used",
+            hint=explain_unavailable(requested, chain),
+        )
     raise errors.TranscribeError(
         "no usable backend remains",
         hint=str(last) if last else "Run `ccn-transcribe doctor` to see what was detected.",

tests/test_backends_registry.py

@@ -225,3 +225,31 @@ class TestRealChain:
             sys.modules.pop(module, None)
         list(registry.candidates())
         assert not {"torch", "faster_whisper", "ctranslate2"} & set(sys.modules)
+
+
+class TestExplicitDeviceDiagnostics:
+    def test_requesting_absent_hardware_is_a_config_error(self) -> None:
+        # A device that cannot run is a configuration mistake, not a per-job
+        # failure to repeat once per URL.
+        a = make_backend("a", hardware=False)
+        with pytest.raises(errors.ConfigError, match="cannot be used"):
+            registry.transcribe_with_fallback(PCM, REQ, requested="a", chain=(a,))
+
+    def test_the_hint_says_the_hardware_is_absent(self) -> None:
+        a = make_backend("a", hardware=False)
+        try:
+            registry.transcribe_with_fallback(PCM, REQ, requested="a", chain=(a,))
+        except errors.ConfigError as exc:
+            assert "no a hardware detected" in (exc.hint or "")
+
+    def test_the_hint_relays_a_toolchain_reason(self) -> None:
+        a = make_backend("a", toolchain=ToolchainStatus(ok=False, reason="not implemented"))
+        assert "not implemented" in registry.explain_unavailable("a", (a,))
+
+    def test_a_runtime_failure_still_reports_as_a_transcribe_error(self) -> None:
+        a = make_backend("a", fail_on_transcribe=True)
+        with pytest.raises(errors.TranscribeError):
+            registry.transcribe_with_fallback(PCM, REQ, requested="a", chain=(a,))
+
+    def test_real_nvidia_request_explains_the_absence(self) -> None:
+        assert "no nvidia hardware" in registry.explain_unavailable("nvidia")