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_translate_posts.py

@@ -5,7 +5,11 @@ from sqlalchemy import select
 
 from backend.app.models import Artist, ImportSettings, Post
 from backend.app.services import interpreter_client as ic
-from backend.app.tasks.translation import retranslate_posts, translate_posts
+from backend.app.tasks.translation import (
+    _RETRANSLATE_COUNTDOWN,
+    retranslate_posts,
+    translate_posts,
+)
 
 pytestmark = pytest.mark.integration
 
@@ -191,6 +195,63 @@ def test_translate_posts_interrupt_reenqueues_after_backoff(db_sync, monkeypatch
     assert calls[0][1]["countdown"] == 30
 
 
+def _mock_ok(monkeypatch):
+    monkeypatch.setattr("backend.app.tasks.translation.ic.health", lambda *a, **k: True)
+    monkeypatch.setattr(
+        "backend.app.tasks.translation.ic.translate",
+        lambda texts, **k: {
+            "translations": [f"EN:{t}" for t in texts],
+            "detected_lang": "ja", "engine": "llm", "engine_version": "v1",
+        },
+    )
+
+
+def test_translate_posts_drain_chases_tail(db_sync, monkeypatch):
+    # "Translate now" (drain=True): with the chunk capped at 1 and 2 untranslated
+    # posts, the first chunk re-enqueues itself — drain preserved — to finish the
+    # backlog rather than waiting for the next beat.
+    _patch(monkeypatch, db_sync)
+    _mock_ok(monkeypatch)
+    monkeypatch.setattr("backend.app.tasks.translation._MAX_POSTS_PER_RUN", 1)
+    calls = []
+    monkeypatch.setattr(
+        "backend.app.tasks.translation.translate_posts.apply_async",
+        lambda *a, **k: calls.append((a, k)),
+    )
+    a = _artist(db_sync)
+    _post(db_sync, a.id, title="ねこ", ext="p1")
+    _post(db_sync, a.id, title="いぬ", ext="p2")
+    _enable(db_sync)
+    db_sync.commit()
+
+    translate_posts(drain=True)
+    assert len(calls) == 1                       # one re-enqueue for the tail
+    args, kw = calls[0]
+    assert args[1] == {"drain": True}            # drain flag carried forward
+    assert kw["countdown"] == _RETRANSLATE_COUNTDOWN
+
+
+def test_translate_posts_beat_single_chunk_no_tail_chase(db_sync, monkeypatch):
+    # The periodic beat (drain defaults False) does exactly ONE chunk and never
+    # re-enqueues — the 8h cadence drives the rest.
+    _patch(monkeypatch, db_sync)
+    _mock_ok(monkeypatch)
+    monkeypatch.setattr("backend.app.tasks.translation._MAX_POSTS_PER_RUN", 1)
+    calls = []
+    monkeypatch.setattr(
+        "backend.app.tasks.translation.translate_posts.apply_async",
+        lambda *a, **k: calls.append((a, k)),
+    )
+    a = _artist(db_sync)
+    _post(db_sync, a.id, title="ねこ", ext="p1")
+    _post(db_sync, a.id, title="いぬ", ext="p2")
+    _enable(db_sync)
+    db_sync.commit()
+
+    translate_posts()                            # drain=False
+    assert calls == []                           # no self-re-enqueue
+
+
 def _mock_new_model(monkeypatch, *, ver="v2"):
     """Interpreter is up and translates with a NEW engine version."""
     monkeypatch.setattr("backend.app.tasks.translation.ic.health", lambda *a, **k: True)