feat(translation): "Test connection" button — on-demand Interpreter health check (#143)

New POST /api/settings/translation/test pings /v1/health for a GIVEN base URL (not
the saved one), so the operator can verify a URL before enabling it. TranslationCard
gains a Test-connection button that reports reachable/unreachable inline and
updates the status dot. Endpoint test.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CDgx8bQS5YrGRK76v8HUnM

tests/test_api_settings.py

@@ -69,6 +69,20 @@ async def test_translation_run_requires_config(client):
     assert resp.status_code == 400   # disabled / no base URL
 
 
+@pytest.mark.asyncio
+async def test_translation_test_connection(client, monkeypatch):
+    # Tests a GIVEN url (not the saved one) without persisting anything.
+    monkeypatch.setattr("backend.app.api.settings.ic.health", lambda *a, **k: True)
+    resp = await client.post(
+        "/api/settings/translation/test", json={"base_url": "http://i.lan"})
+    assert resp.status_code == 200
+    assert (await resp.get_json())["healthy"] is True
+    # Empty URL → False, no health call.
+    empty = await client.post(
+        "/api/settings/translation/test", json={"base_url": ""})
+    assert (await empty.get_json())["healthy"] is False
+
+
 @pytest.mark.asyncio
 async def test_translation_run_enqueues_when_configured(client, monkeypatch):
     monkeypatch.setattr(