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

backend/app/api/settings.py

@@ -364,9 +364,49 @@ async def translation_test():
     return jsonify({"healthy": healthy})
 
 
+@settings_bp.route("/settings/translation/probe", methods=["POST"])
+async def translation_probe():
+    """Diagnostic for the Settings 'Test translation' box: translate a pasted
+    snippet WITHOUT saving anything, returning what Interpreter *detected*
+    (language + confidence) alongside the result. Lets the operator see why a
+    given string was (mis-)detected — e.g. a short English title flagged as
+    another language — so a detection guard can be tuned from real numbers.
+    Read-only: no post is touched. Uses the currently-saved base URL + target."""
+    body = await request.get_json(silent=True)
+    body = body if isinstance(body, dict) else {}
+    text = (body.get("text") or "").strip()
+    if not text:
+        return jsonify({"error": "provide text to translate"}), 400
+    async with get_session() as session:
+        cfg = await ImportSettings.load(session)
+    base_url = (cfg.interpreter_base_url or "").strip()
+    if not base_url:
+        return jsonify({"error": "no Interpreter base URL is set"}), 400
+    target = (cfg.translation_target_lang or "en").strip() or "en"
+    try:
+        res = await asyncio.to_thread(
+            ic.translate, [text], base_url=base_url, target=target,
+        )
+    except ic.InterpreterUnavailable as e:
+        return jsonify({"error": f"Interpreter unavailable: {e}"}), 503
+    except ic.InterpreterBadRequest as e:
+        return jsonify({"error": f"Interpreter rejected the request: {e}"}), 400
+    translations = res.get("translations") or []
+    return jsonify({
+        "target": target,
+        "detected_lang": res.get("detected_lang"),
+        "detected_confidence": res.get("detected_confidence"),
+        "engine": res.get("engine"),
+        "engine_version": res.get("engine_version"),
+        "translated": translations[0] if translations else None,
+    })
+
+
 @settings_bp.route("/settings/translation/run", methods=["POST"])
 async def translation_run():
-    """Enqueue the translate sweep now (the Settings 'Translate now' button)."""
+    """Enqueue the translate sweep now (the Settings 'Translate now' button).
+    Runs in drain mode — run-until-done — so one press chases the whole
+    untranslated backlog to zero rather than a single 300-post chunk."""
     async with get_session() as session:
         cfg = await ImportSettings.load(session)
     if not cfg.translation_enabled or not (cfg.interpreter_base_url or "").strip():
@@ -375,7 +415,7 @@ async def translation_run():
         ), 400
     from ..tasks.translation import translate_posts
 
-    r = translate_posts.delay()
+    r = translate_posts.delay(drain=True)
     return jsonify({"celery_task_id": r.id}), 202