feat(translation): 8h sweep + drain button + detection probe (#1376)

Throughput: translate_posts now runs every 8h (was daily) as the
steady-state cadence for newly-imported posts, and the Settings
"Translate now" button runs it in drain mode (run-until-done, no reset)
so one press clears the whole untranslated backlog instead of a single
300-post chunk. The interrupt/backoff re-enqueue now preserves the drain
flag so a bulk drain resumes cleanly after an Interpreter restart.

Misdetection groundwork: surface the detector's confidence from the
Interpreter client (it was in the detectedLanguage payload but discarded)
and add a read-only "Test translation" box — POST /settings/translation/
probe + TranslationCard UI — that shows detected language + confidence +
engine + result for pasted text, without saving. Lets the operator see
why a short/abbreviation-heavy English title gets mis-detected so the
detection guard (min-length + confidence floor) can be tuned from real
numbers. The guard itself follows once the mis-detected cases are probed.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CgZP9v2otxVJymiYsnVuMy

tests/test_api_settings.py

@@ -113,9 +113,10 @@ async def test_translation_test_connection(client, monkeypatch):
 
 @pytest.mark.asyncio
 async def test_translation_run_enqueues_when_configured(client, monkeypatch):
+    sink = {}
     monkeypatch.setattr(
         "backend.app.tasks.translation.translate_posts.delay",
-        lambda *a, **k: type("R", (), {"id": "t1"})(),
+        lambda *a, **k: sink.update(k) or type("R", (), {"id": "t1"})(),
     )
     await client.patch("/api/settings/import", json={
         "translation_enabled": True, "interpreter_base_url": "http://i.lan",
@@ -123,6 +124,46 @@ async def test_translation_run_enqueues_when_configured(client, monkeypatch):
     resp = await client.post("/api/settings/translation/run")
     assert resp.status_code == 202
     assert (await resp.get_json())["celery_task_id"] == "t1"
+    assert sink["drain"] is True   # "Translate now" chases the whole backlog
+
+
+@pytest.mark.asyncio
+async def test_translation_probe_requires_text(client):
+    resp = await client.post(
+        "/api/settings/translation/probe", json={"text": "  "})
+    assert resp.status_code == 400
+
+
+@pytest.mark.asyncio
+async def test_translation_probe_requires_base_url(client):
+    # Text given but no Interpreter URL saved → nothing to probe against.
+    resp = await client.post(
+        "/api/settings/translation/probe", json={"text": "hello"})
+    assert resp.status_code == 400
+
+
+@pytest.mark.asyncio
+async def test_translation_probe_returns_detection(client, monkeypatch):
+    # The diagnostic surfaces detected language + confidence + result, unsaved.
+    monkeypatch.setattr(
+        "backend.app.api.settings.ic.translate",
+        lambda texts, **k: {
+            "translations": ["Work in Progress"],
+            "detected_lang": "de", "detected_confidence": 71.5,
+            "engine": "llm", "engine_version": "v1",
+        },
+    )
+    await client.patch("/api/settings/import", json={
+        "translation_enabled": True, "interpreter_base_url": "http://i.lan",
+    })
+    resp = await client.post(
+        "/api/settings/translation/probe", json={"text": "WIP"})
+    assert resp.status_code == 200
+    body = await resp.get_json()
+    assert body["detected_lang"] == "de"
+    assert body["detected_confidence"] == 71.5
+    assert body["translated"] == "Work in Progress"
+    assert body["target"] == "en"
 
 
 def _fake_retranslate(monkeypatch, sink):