feat(translation): per-field language detection for mixed-language posts

_translate_one translated [title, description] in ONE Interpreter call and keyed
the whole-post passthrough on the aggregate detected_lang (the FIRST item). So an
English title + non-English description detected "en" and marked the post handled,
leaving the description untranslated. Now each field is translated independently
(its own detected_lang / passthrough) and the non-target field is stored on its
own; translated_source_lang reflects the translated field's language.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_translate_posts.py

@@ -115,6 +115,35 @@ def test_translate_posts_passthrough_english_marks_handled(db_sync, monkeypatch)
     assert p.post_title_translated is None       # ...but nothing to show
 
 
+def test_translate_posts_mixed_language_translates_nonenglish_field(db_sync, monkeypatch):
+    # English title + Japanese description: the description is still translated
+    # even though the title is already English (per-field detection, not the old
+    # aggregate first-item bail). Source lang comes from the translated field.
+    _patch(monkeypatch, db_sync)
+    monkeypatch.setattr("backend.app.tasks.translation.ic.health", lambda *a, **k: True)
+
+    def fake_translate(texts, **k):
+        t = texts[0]
+        if t.isascii():   # already English → passthrough
+            return {"translations": [t], "detected_lang": "en",
+                    "engine": "none", "engine_version": None}
+        return {"translations": [f"EN:{t}"], "detected_lang": "ja",
+                "engine": "llm", "engine_version": "v9"}
+
+    monkeypatch.setattr("backend.app.tasks.translation.ic.translate", fake_translate)
+    a = _artist(db_sync)
+    p = _post(db_sync, a.id, title="hello", desc="ねこ")
+    _enable(db_sync)
+    db_sync.commit()
+
+    assert "translated=1" in translate_posts()
+    db_sync.refresh(p)
+    assert p.post_title_translated is None          # English title untouched
+    assert p.description_translated == "EN:ねこ"      # Japanese desc translated
+    assert p.translated_source_lang == "ja"          # from the translated field
+    assert p.translation_engine_version == "v9"
+
+
 def test_translate_posts_disabled_is_noop(db_sync, monkeypatch):
     _patch(monkeypatch, db_sync)
     a = _artist(db_sync)