feat(translation): tunable acceptance floor (0.90) + per-post sticky override (#155)

The gate at a fixed 0.80 couldn't catch the real pain: Interpreter (fresh ==
cached, verified by probe) confidently mis-detects short ASCII English like
"... WIP Part 1" as German at 0.86 — above the floor — so it was accepted and a
re-translate reproduced it. Confidence alone can't separate the 0.86 collision
(genuine German lands there too), and single-word mis-flags sit at a confident
1.0 no floor catches.

Two operator-approved levers:

- Acceptance floor is now a live Settings value (ImportSettings.
  translation_min_confidence, default 0.90; surfaced in the Translation card), so
  it's tunable without a redeploy. _accept takes the threshold as a parameter.

- Per-post sticky override (Post.translation_override: auto/force/original).
  'force' stores a translation even below the floor (rescue a skipped
  legit-foreign title); 'original' keeps the original and clears any stored
  translation (kill a confident mis-flag no floor catches). The sweep honors it
  on every run and _reset_translations skips 'original', so the choice survives a
  Re-translate-all. POST /api/posts/<id>/translation-override applies it
  immediately (translate now when the service is up, else queue for the sweep).
  UI: PostTranslationControl on the posts-feed card.

Migration 0084 (both columns + a CHECK on the override). The feed + provenance
serializers expose translation_override.

With a stricter floor the rollback finally works: raise it -> Re-translate all ->
the 0.86 mis-flags are rejected and restored to the original; force /
keep-original handle the residual either way.

Tests: gate thresholds against the param (0.86 rejected at 0.90, explicit-floor
cases); sweep force/original + re-translate-skips-original; override endpoint
(validation, original clears, force queues when disabled, feed exposes it);
settings min_confidence default/save/validate.

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

@@ -314,6 +314,41 @@ def test_translate_posts_accepts_low_confidence_cjk(db_sync, monkeypatch):
     assert p.translated_source_lang == "zh"
 
 
+def test_translate_posts_force_override_translates_below_floor(db_sync, monkeypatch):
+    # override='force' stores the translation even below the confidence floor —
+    # rescuing a legitimately-foreign title the gate would otherwise skip.
+    _patch(monkeypatch, db_sync)
+    _mock_lang(monkeypatch, lang="de", conf=0.42)     # below the 0.90 default floor
+    a = _artist(db_sync)
+    p = _post(db_sync, a.id, title="Kurz", ext="p1")
+    p.translation_override = "force"
+    _enable(db_sync)
+    db_sync.commit()
+
+    translate_posts()
+    db_sync.refresh(p)
+    assert p.post_title_translated == "EN:Kurz"        # forced through despite 0.42
+    assert p.translated_source_lang == "de"
+
+
+def test_translate_posts_original_override_keeps_original(db_sync, monkeypatch):
+    # override='original' never translates — even a confident (0.98) detection is
+    # kept as the original. The sweep marks it handled (== target) and stores
+    # nothing; Interpreter isn't consulted for the decision.
+    _patch(monkeypatch, db_sync)
+    _mock_lang(monkeypatch, lang="de", conf=0.98)     # would be accepted under auto
+    a = _artist(db_sync)
+    p = _post(db_sync, a.id, title="Das Mädchen", ext="p1")
+    p.translation_override = "original"
+    _enable(db_sync)
+    db_sync.commit()
+
+    translate_posts()
+    db_sync.refresh(p)
+    assert p.post_title_translated is None             # kept original
+    assert p.translated_source_lang == "en"            # marked handled
+
+
 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)
@@ -344,6 +379,25 @@ def test_retranslate_resets_and_reruns(db_sync, monkeypatch):
     assert p.translation_engine_version == "v2"
 
 
+def test_retranslate_leaves_keep_original_posts(db_sync, monkeypatch):
+    # A 'keep original' post survives a Re-translate-all: _reset_translations skips
+    # override='original', so it stays handled + untranslated instead of being
+    # re-run (which would re-introduce the mis-flag the operator killed).
+    _patch(monkeypatch, db_sync)
+    _mock_new_model(monkeypatch)
+    a = _artist(db_sync)
+    p = _post(db_sync, a.id, title="ねこ", ext="p1")
+    p.translation_override = "original"
+    p.translated_source_lang = "en"       # handled by a prior 'keep original' apply
+    _enable(db_sync)
+    db_sync.commit()
+
+    retranslate_posts(artist_ids=[a.id])
+    db_sync.refresh(p)
+    assert p.translated_source_lang == "en"     # not reset
+    assert p.post_title_translated is None       # not re-translated
+
+
 def test_retranslate_artist_scope_leaves_others(db_sync, monkeypatch):
     # Scoping to one artist must not touch another artist's translations.
     _patch(monkeypatch, db_sync)