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_interpreter_client.py

@@ -34,6 +34,7 @@ def test_translate_maps_batch_in_order(monkeypatch):
     out = ic.translate(["ねこが可愛い", "金髪ギャル！"], base_url="http://i.lan")
     assert out["translations"] == ["The cat is cute", "Blonde gal!"]
     assert out["detected_lang"] == "ja"
+    assert out["detected_confidence"] == 0.98   # surfaced for the detection guard
     assert out["engine_version"] == "ollama:x:12b"
     assert captured["json"]["q"] == ["ねこが可愛い", "金髪ギャル！"]
     assert captured["json"]["target"] == "en"