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/services/interpreter_client.py

@@ -96,8 +96,9 @@ def translate(
 ) -> dict:
     """Translate a batch. Returns::
 
-        {"translations": [str, ...],   # SAME order & length as `texts`
-         "detected_lang": str | None,  # aggregate: describes the FIRST item
+        {"translations": [str, ...],        # SAME order & length as `texts`
+         "detected_lang": str | None,       # aggregate: describes the FIRST item
+         "detected_confidence": float | None,  # detector's confidence, if given
          "engine": str | None,
          "engine_version": str | None}
 
@@ -110,6 +111,7 @@ def translate(
     """
     if not texts:
         return {"translations": [], "detected_lang": None,
+                "detected_confidence": None,
                 "engine": None, "engine_version": None}
     try:
         r = session.post(
@@ -140,9 +142,11 @@ def translate(
     if not isinstance(out, list):
         out = [out]
     interp = body.get("interpreter") or {}
+    detected = body.get("detectedLanguage") or {}
     return {
         "translations": out,
-        "detected_lang": (body.get("detectedLanguage") or {}).get("language"),
+        "detected_lang": detected.get("language"),
+        "detected_confidence": detected.get("confidence"),
         "engine": interp.get("engine"),
         "engine_version": interp.get("engine_version"),
     }